diff --git a/CHANGELOG.md b/CHANGELOG.md
index 417ccd77..e9300d77 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,6 @@
 # CHANGELOG
 
+  * Added support for plain C11 translation units (#97)
   * Added 'row' and 'column' layout hints for aligning elements (#90)
   * Added 'together' layout hint for grouping elements (#43)
   * Enabled adding notes to class methods and members (#87)
diff --git a/README.md b/README.md
index c55653d8..da7caf82 100644
--- a/README.md
+++ b/README.md
@@ -28,6 +28,7 @@ Main features supported so far include:
     * Diagram content filtering based on namespaces, elements and relationships
     * Optional package generation from namespaces
     * Interactive links to online code to classes, methods and class fields in SVG diagrams
+    * Support for plain C99/C11 code (struct and units relationships)
 * **Sequence diagram generation**
     * Generation of sequence diagram from specific method or function
     * Generation of loop and conditional statements
diff --git a/src/class_diagram/generators/plantuml/class_diagram_generator.cc b/src/class_diagram/generators/plantuml/class_diagram_generator.cc
index 6c3b0541..8b18db8b 100644
--- a/src/class_diagram/generators/plantuml/class_diagram_generator.cc
+++ b/src/class_diagram/generators/plantuml/class_diagram_generator.cc
@@ -116,6 +116,10 @@ void generator::generate(const class_ &c, std::ostream &ostr) const
 
     ostr << class_type << " " << c.alias();
 
+    if (c.is_union())
+        ostr << " "
+             << "<<union>>";
+
     if (m_config.generate_links) {
         common_generator<diagram_config, diagram_model>::generate_link(ostr, c);
     }
diff --git a/src/class_diagram/model/class.h b/src/class_diagram/model/class.h
index 0fcbfa76..97b55e19 100644
--- a/src/class_diagram/model/class.h
+++ b/src/class_diagram/model/class.h
@@ -54,6 +54,12 @@ public:
     bool is_template_instantiation() const;
     void is_template_instantiation(bool is_template_instantiation);
 
+    bool is_alias() const { return is_alias_; }
+    void is_alias(bool alias) { is_alias_ = alias; }
+
+    bool is_union() const { return is_union_; }
+    void is_union(bool u) { is_union_ = u; }
+
     void add_member(class_member &&member);
     void add_method(class_method &&method);
     void add_parent(class_parent &&parent);
@@ -73,10 +79,6 @@ public:
 
     bool is_abstract() const;
 
-    bool is_alias() const { return is_alias_; }
-
-    void is_alias(bool alias) { is_alias_ = alias; }
-
     void find_relationships(
         std::vector<std::pair<std::string, common::model::relationship_t>>
             &nested_relationships);
@@ -89,6 +91,7 @@ private:
     bool is_template_{false};
     bool is_template_instantiation_{false};
     bool is_alias_{false};
+    bool is_union_{false};
     std::vector<class_member> members_;
     std::vector<class_method> methods_;
     std::vector<class_parent> bases_;
diff --git a/src/class_diagram/visitor/translation_unit_visitor.cc b/src/class_diagram/visitor/translation_unit_visitor.cc
index 99071c7e..62ddcafa 100644
--- a/src/class_diagram/visitor/translation_unit_visitor.cc
+++ b/src/class_diagram/visitor/translation_unit_visitor.cc
@@ -310,6 +310,63 @@ bool translation_unit_visitor::VisitClassTemplateDecl(
     return true;
 }
 
+bool translation_unit_visitor::VisitRecordDecl(clang::RecordDecl *rec)
+{
+    // Skip system headers
+    if (source_manager().isInSystemHeader(rec->getSourceRange().getBegin()))
+        return true;
+
+    if (clang::dyn_cast_or_null<clang::CXXRecordDecl>(rec))
+        // This is handled by VisitCXXRecordDecl()
+        return true;
+
+    // It seems we are in a C (not C++) translation unit
+    if (!diagram().should_include(rec->getQualifiedNameAsString()))
+        return true;
+
+    LOG_DBG("= Visiting record declaration {} at {}",
+        rec->getQualifiedNameAsString(),
+        rec->getLocation().printToString(source_manager()));
+
+    auto record_ptr = create_record_declaration(rec);
+
+    if (!record_ptr)
+        return true;
+
+    const auto rec_id = record_ptr->id();
+
+    set_ast_local_id(rec->getID(), rec_id);
+
+    auto &record_model = diagram().get_class(rec_id).has_value()
+        ? *diagram().get_class(rec_id).get()
+        : *record_ptr;
+
+    if (rec->isCompleteDefinition() && !record_model.complete()) {
+        process_record_members(rec, record_model);
+        record_model.complete(true);
+    }
+
+    auto id = record_model.id();
+    if (!rec->isCompleteDefinition()) {
+        forward_declarations_.emplace(id, std::move(record_ptr));
+        return true;
+    }
+    forward_declarations_.erase(id);
+
+    if (diagram_.should_include(record_model)) {
+        LOG_DBG("Adding struct/union {} with id {}",
+            record_model.full_name(false), record_model.id());
+
+        diagram_.add_class(std::move(record_ptr));
+    }
+    else {
+        LOG_DBG("Skipping struct/union {} with id {}", record_model.full_name(),
+            record_model.id());
+    }
+
+    return true;
+}
+
 bool translation_unit_visitor::VisitCXXRecordDecl(clang::CXXRecordDecl *cls)
 {
     // Skip system headers
@@ -383,6 +440,42 @@ bool translation_unit_visitor::VisitCXXRecordDecl(clang::CXXRecordDecl *cls)
     return true;
 }
 
+std::unique_ptr<class_> translation_unit_visitor::create_record_declaration(
+    clang::RecordDecl *rec)
+{
+    assert(rec != nullptr);
+
+    auto record_ptr{std::make_unique<class_>(config_.using_namespace())};
+    auto &record = *record_ptr;
+
+    auto qualified_name = rec->getQualifiedNameAsString();
+
+    if (!diagram().should_include(qualified_name))
+        return {};
+
+    process_record_parent(rec, record, namespace_{});
+
+    if (!record.is_nested()) {
+        record.set_name(common::get_tag_name(*rec));
+        record.set_id(common::to_id(record.full_name(false)));
+    }
+
+    process_comment(*rec, record);
+    set_source_location(*rec, record);
+
+    const auto record_full_name = record_ptr->full_name(false);
+
+    record.is_struct(rec->isStruct());
+    record.is_union(rec->isUnion());
+
+    if (record.skip())
+        return {};
+
+    record.set_style(record.style_spec());
+
+    return record_ptr;
+}
+
 std::unique_ptr<class_> translation_unit_visitor::create_class_declaration(
     clang::CXXRecordDecl *cls)
 {
@@ -392,14 +485,37 @@ std::unique_ptr<class_> translation_unit_visitor::create_class_declaration(
     auto &c = *c_ptr;
 
     // TODO: refactor to method get_qualified_name()
-    auto qualified_name =
-        cls->getQualifiedNameAsString(); //  common::get_qualified_name(*cls);
+    auto qualified_name = cls->getQualifiedNameAsString();
 
     if (!diagram().should_include(qualified_name))
         return {};
 
     auto ns = common::get_tag_namespace(*cls);
 
+    process_record_parent(cls, c, ns);
+
+    if (!c.is_nested()) {
+        c.set_name(common::get_tag_name(*cls));
+        c.set_namespace(ns);
+        c.set_id(common::to_id(c.full_name(false)));
+    }
+
+    c.is_struct(cls->isStruct());
+
+    process_comment(*cls, c);
+    set_source_location(*cls, c);
+
+    if (c.skip())
+        return {};
+
+    c.set_style(c.style_spec());
+
+    return c_ptr;
+}
+
+void translation_unit_visitor::process_record_parent(
+    clang::RecordDecl *cls, class_ &c, const namespace_ &ns)
+{
     const auto *parent = cls->getParent();
 
     std::optional<common::model::diagram_element::id_t> id_opt;
@@ -435,7 +551,8 @@ std::unique_ptr<class_> translation_unit_visitor::create_class_declaration(
         assert(parent_class);
 
         c.set_namespace(ns);
-        if (cls->getNameAsString().empty()) {
+        const auto cls_name = cls->getNameAsString();
+        if (cls_name.empty()) {
             // Nested structs can be anonymous
             if (anonymous_struct_relationships_.count(cls->getID()) > 0) {
                 const auto &[label, hint, access] =
@@ -467,23 +584,6 @@ std::unique_ptr<class_> translation_unit_visitor::create_class_declaration(
 
         c.nested(true);
     }
-    else {
-        c.set_name(common::get_tag_name(*cls));
-        c.set_namespace(ns);
-        c.set_id(common::to_id(c.full_name(false)));
-    }
-
-    c.is_struct(cls->isStruct());
-
-    process_comment(*cls, c);
-    set_source_location(*cls, c);
-
-    if (c.skip())
-        return {};
-
-    c.set_style(c.style_spec());
-
-    return c_ptr;
 }
 
 void translation_unit_visitor::process_class_declaration(
@@ -715,6 +815,16 @@ void translation_unit_visitor::process_template_specialization_children(
     }
 }
 
+void translation_unit_visitor::process_record_members(
+    const clang::RecordDecl *cls, class_ &c)
+{
+    // Iterate over regular class fields
+    for (const auto *field : cls->fields()) {
+        if (field != nullptr)
+            process_field(*field, c);
+    }
+}
+
 void translation_unit_visitor::process_class_children(
     const clang::CXXRecordDecl *cls, class_ &c)
 {
@@ -1054,11 +1164,16 @@ bool translation_unit_visitor::find_relationships(const clang::QualType &type,
                 }
             }
         }
-        else {
+        else if (type->getAsCXXRecordDecl()) {
             const auto target_id = common::to_id(*type->getAsCXXRecordDecl());
             relationships.emplace_back(target_id, relationship_hint);
             result = true;
         }
+        else {
+            const auto target_id = common::to_id(*type->getAsRecordDecl());
+            relationships.emplace_back(target_id, relationship_hint);
+            result = true;
+        }
     }
 
     return result;
@@ -2092,6 +2207,9 @@ bool translation_unit_visitor::build_template_instantiation_add_base_classes(
 void translation_unit_visitor::process_field(
     const clang::FieldDecl &field_declaration, class_ &c)
 {
+    LOG_DBG(
+        "== Visiting record member {}", field_declaration.getNameAsString());
+
     // Default hint for relationship is aggregation
     auto relationship_hint = relationship_t::kAggregation;
     // If the first type of the template instantiation of this field type
@@ -2108,6 +2226,7 @@ void translation_unit_visitor::process_field(
 
     auto field_type_str =
         common::to_string(field_type, field_declaration.getASTContext(), false);
+
     ensure_lambda_type_is_relative(field_type_str);
 
     class_member field{
@@ -2239,11 +2358,11 @@ void translation_unit_visitor::process_field(
         // Find relationship for the type if the type has not been added
         // as aggregation
         if (!template_instantiation_added_as_aggregation) {
-            if ((field_type->getAsCXXRecordDecl() != nullptr) &&
-                field_type->getAsCXXRecordDecl()->getNameAsString().empty()) {
+            if ((field_type->getAsRecordDecl() != nullptr) &&
+                field_type->getAsRecordDecl()->getNameAsString().empty()) {
                 // Relationships to fields whose type is an anonymous nested
                 // struct have to be handled separately here
-                anonymous_struct_relationships_[field_type->getAsCXXRecordDecl()
+                anonymous_struct_relationships_[field_type->getAsRecordDecl()
                                                     ->getID()] =
                     std::make_tuple(
                         field.name(), relationship_hint, field.access());
diff --git a/src/class_diagram/visitor/translation_unit_visitor.h b/src/class_diagram/visitor/translation_unit_visitor.h
index 9313087c..d275c2e8 100644
--- a/src/class_diagram/visitor/translation_unit_visitor.h
+++ b/src/class_diagram/visitor/translation_unit_visitor.h
@@ -69,6 +69,8 @@ public:
 
     virtual bool VisitNamespaceDecl(clang::NamespaceDecl *ns);
 
+    virtual bool VisitRecordDecl(clang::RecordDecl *D);
+
     virtual bool VisitCXXRecordDecl(clang::CXXRecordDecl *d);
 
     virtual bool VisitEnumDecl(clang::EnumDecl *e);
@@ -109,6 +111,9 @@ private:
     std::unique_ptr<clanguml::class_diagram::model::class_>
     create_class_declaration(clang::CXXRecordDecl *cls);
 
+    std::unique_ptr<clanguml::class_diagram::model::class_>
+    create_record_declaration(clang::RecordDecl *rec);
+
     void process_class_declaration(const clang::CXXRecordDecl &cls,
         clanguml::class_diagram::model::class_ &c);
 
@@ -118,6 +123,8 @@ private:
     void process_class_children(const clang::CXXRecordDecl *cls,
         clanguml::class_diagram::model::class_ &c);
 
+    void process_record_members(const clang::RecordDecl *cls, class_ &c);
+
     std::unique_ptr<clanguml::class_diagram::model::class_>
     process_template_specialization(
         clang::ClassTemplateSpecializationDecl *cls);
@@ -224,6 +231,9 @@ private:
 
     void ensure_lambda_type_is_relative(std::string &parameter_type) const;
 
+    void process_record_parent(
+        clang::RecordDecl *cls, class_ &c, const namespace_ &ns);
+
     void process_function_parameter_find_relatinoships_in_autotype(
         model::class_ &c, const clang::AutoType *atsp);
 
diff --git a/src/common/clang_utils.cc b/src/common/clang_utils.cc
index 57cea568..ce94c1ff 100644
--- a/src/common/clang_utils.cc
+++ b/src/common/clang_utils.cc
@@ -152,7 +152,12 @@ std::string to_string(const clang::QualType &type, const clang::ASTContext &ctx,
     // it has some default name
     if (result.empty())
         result = "(anonymous)";
-    else if (util::contains(result, "unnamed struct")) {
+    else if (util::contains(result, "unnamed struct") ||
+        util::contains(result, "unnamed union")) {
+        result = common::get_tag_name(*type->getAsTagDecl());
+    }
+    else if (util::contains(result, "anonymous struct") ||
+        util::contains(result, "anonymous union")) {
         result = common::get_tag_name(*type->getAsTagDecl());
     }
 
diff --git a/src/common/clang_utils.h b/src/common/clang_utils.h
index 1c162ae5..6e40ee6d 100644
--- a/src/common/clang_utils.h
+++ b/src/common/clang_utils.h
@@ -125,6 +125,8 @@ template <> id_t to_id(const clang::NamespaceDecl &declaration);
 
 template <> id_t to_id(const clang::CXXRecordDecl &declaration);
 
+template <> id_t to_id(const clang::RecordDecl &declaration);
+
 template <> id_t to_id(const clang::EnumDecl &declaration);
 
 template <> id_t to_id(const clang::TagDecl &declaration);
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index ded8a48b..f9bf22c5 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -1,5 +1,5 @@
 
-file(GLOB_RECURSE TEST_CASE_SOURCES t*/*.cc)
+file(GLOB_RECURSE TEST_CASE_SOURCES t*/*.cc t*/*.c t*/src/*.c)
 file(GLOB_RECURSE TEST_CASE_CONFIGS t*/.clang-uml)
 file(GLOB_RECURSE TEST_CONFIG_YMLS test_config_data/*.yml)
 
diff --git a/tests/t00057/.clang-uml b/tests/t00057/.clang-uml
new file mode 100644
index 00000000..6ce6b2aa
--- /dev/null
+++ b/tests/t00057/.clang-uml
@@ -0,0 +1,8 @@
+compilation_database_dir: ..
+output_directory: puml
+diagrams:
+  t00057_class:
+    type: class
+    glob:
+      - ../../tests/t00057/t00057.c
+      - ../../tests/t00057/src/t00057_impl.c
\ No newline at end of file
diff --git a/tests/t00057/include/t00057.h b/tests/t00057/include/t00057.h
new file mode 100644
index 00000000..129663bf
--- /dev/null
+++ b/tests/t00057/include/t00057.h
@@ -0,0 +1,3 @@
+#pragma once
+
+struct t00057_F;
\ No newline at end of file
diff --git a/tests/t00057/src/t00057_impl.c b/tests/t00057/src/t00057_impl.c
new file mode 100644
index 00000000..c7a4071c
--- /dev/null
+++ b/tests/t00057/src/t00057_impl.c
@@ -0,0 +1,5 @@
+#include "../include/t00057.h"
+
+struct t00057_F {
+    int f1;
+};
\ No newline at end of file
diff --git a/tests/t00057/t00057.c b/tests/t00057/t00057.c
new file mode 100644
index 00000000..2c025cd4
--- /dev/null
+++ b/tests/t00057/t00057.c
@@ -0,0 +1,39 @@
+#include "include/t00057.h"
+
+struct t00057_A {
+    int a1;
+};
+
+typedef struct t00057_B {
+    int b1;
+} t00057_B;
+
+struct t00057_C {
+    int c1;
+};
+
+union t00057_D {
+    int d1;
+    float d2;
+};
+
+struct t00057_E {
+    int e;
+    struct {
+        int x;
+        int y;
+    } coordinates;
+    union {
+        int z;
+        double t;
+    } height;
+};
+
+struct t00057_R {
+    struct t00057_A a;
+    t00057_B b;
+    struct t00057_C *c;
+    union t00057_D d;
+    struct t00057_E *e;
+    struct t00057_F *f;
+};
diff --git a/tests/t00057/test_case.h b/tests/t00057/test_case.h
new file mode 100644
index 00000000..02b89d93
--- /dev/null
+++ b/tests/t00057/test_case.h
@@ -0,0 +1,60 @@
+/**
+ * tests/t00057/test_case.h
+ *
+ * Copyright (c) 2021-2023 Bartek Kryza <bkryza@gmail.com>
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+TEST_CASE("t00057", "[test-case][class]")
+{
+    auto [config, db] = load_config("t00057");
+
+    auto diagram = config.diagrams["t00057_class"];
+
+    REQUIRE(diagram->name == "t00057_class");
+
+    auto model = generate_class_diagram(*db, diagram);
+
+    REQUIRE(model->name() == "t00057_class");
+
+    auto puml = generate_class_puml(diagram, *model);
+    AliasMatcher _A(puml);
+
+    REQUIRE_THAT(puml, StartsWith("@startuml"));
+    REQUIRE_THAT(puml, EndsWith("@enduml\n"));
+
+    // Check if all classes exist
+    REQUIRE_THAT(puml, IsClass(_A("t00057_A")));
+    REQUIRE_THAT(puml, IsClass(_A("t00057_B")));
+    REQUIRE_THAT(puml, IsClass(_A("t00057_C")));
+    REQUIRE_THAT(puml, IsUnion(_A("t00057_D")));
+    REQUIRE_THAT(puml, IsClass(_A("t00057_E")));
+    REQUIRE_THAT(puml, IsClass(_A("t00057_F")));
+    REQUIRE_THAT(puml, IsClass(_A("t00057_R")));
+
+    // Check if all relationships exist
+    REQUIRE_THAT(puml, IsAggregation(_A("t00057_R"), _A("t00057_A"), "+a"));
+    REQUIRE_THAT(puml, IsAggregation(_A("t00057_R"), _A("t00057_B"), "+b"));
+    REQUIRE_THAT(puml, IsAssociation(_A("t00057_R"), _A("t00057_C"), "+c"));
+    REQUIRE_THAT(puml, IsAggregation(_A("t00057_R"), _A("t00057_D"), "+d"));
+    REQUIRE_THAT(puml, IsAssociation(_A("t00057_R"), _A("t00057_E"), "+e"));
+    REQUIRE_THAT(puml, IsAssociation(_A("t00057_R"), _A("t00057_F"), "+f"));
+    REQUIRE_THAT(puml,
+        IsAggregation(
+            _A("t00057_E"), _A("t00057_E::(coordinates)"), "+coordinates"));
+    REQUIRE_THAT(puml,
+        IsAggregation(_A("t00057_E"), _A("t00057_E::(height)"), "+height"));
+
+    save_puml(config.output_directory() + "/" + diagram->name + ".puml", puml);
+}
\ No newline at end of file
diff --git a/tests/test_cases.cc b/tests/test_cases.cc
index c8e8ef3b..a1d4d618 100644
--- a/tests/test_cases.cc
+++ b/tests/test_cases.cc
@@ -255,6 +255,7 @@ using namespace clanguml::test::matchers;
 #include "t00053/test_case.h"
 #include "t00054/test_case.h"
 #include "t00055/test_case.h"
+#include "t00057/test_case.h"
 
 ///
 /// Sequence diagram tests
diff --git a/tests/test_cases.h b/tests/test_cases.h
index 388e6ae7..99894176 100644
--- a/tests/test_cases.h
+++ b/tests/test_cases.h
@@ -266,6 +266,13 @@ ContainsMatcher IsClass(std::string const &str,
     return ContainsMatcher(CasedString("class " + str, caseSensitivity));
 }
 
+ContainsMatcher IsUnion(std::string const &str,
+    CaseSensitive::Choice caseSensitivity = CaseSensitive::Yes)
+{
+    return ContainsMatcher(
+        CasedString("class " + str + " <<union>>", caseSensitivity));
+}
+
 ContainsMatcher IsClassTemplate(std::string const &str,
     std::string const &tmplt,
     CaseSensitive::Choice caseSensitivity = CaseSensitive::Yes)
diff --git a/tests/test_cases.yaml b/tests/test_cases.yaml
index df742152..0cbdbea1 100644
--- a/tests/test_cases.yaml
+++ b/tests/test_cases.yaml
@@ -162,6 +162,9 @@ test_cases:
     - name: t00055
       title: Test case for `row` and `column` layout hints
       description:
+    - name: t00057
+      title: Test case C99/C11 translation units with structs and unions
+      description:
   Sequence diagrams:
     - name: t20001
       title: Basic sequence diagram test case