diff --git a/docs/design/definition.yaml b/docs/design/definition.yaml
index 03c70bb..ed31baa 100644
--- a/docs/design/definition.yaml
+++ b/docs/design/definition.yaml
@@ -4,6 +4,7 @@ resource-path:
   - docs/template
 input-files:
   - docs/design/title.txt
+  - docs/design/introduction.md
   - docs/design/file-assert-rule.md
   - docs/design/file-assert-file.md
   - docs/design/file-assert-test.md
diff --git a/docs/design/introduction.md b/docs/design/introduction.md
new file mode 100644
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+++ b/docs/design/introduction.md
@@ -0,0 +1,100 @@
+# Introduction
+
+This document provides the detailed design for the FileAssert tool, a .NET command-line application
+for asserting file properties using YAML-defined test suites.
+
+## Purpose
+
+The purpose of this document is to describe the internal design of each software unit that comprises
+FileAssert. It captures data models, algorithms, key methods, and inter-unit interactions at a level
+of detail sufficient for formal code review, compliance verification, and future maintenance. The
+document does not restate requirements; it explains how they are realized.
+
+## Scope
+
+This document covers the detailed design of the following software units:
+
+- **Program** — entry point and execution orchestrator (`Program.cs`)
+- **Context** — command-line argument parser and I/O owner (`Context.cs`)
+- **FileAssertConfig** — top-level configuration loader and test runner (`FileAssertConfig.cs`)
+- **FileAssertData** — YAML data transfer objects for configuration deserialization (`FileAssertData.cs`)
+- **FileAssertTest** — named test with file assertions and tag filtering (`FileAssertTest.cs`)
+- **FileAssertFile** — glob pattern matcher with count constraints and content rules (`FileAssertFile.cs`)
+- **FileAssertRule** — abstract content validation rule hierarchy (`FileAssertRule.cs`)
+- **PathHelpers** — safe path-combination utility (`PathHelpers.cs`)
+- **Validation** — self-validation test runner (`Validation.cs`)
+
+The following topics are out of scope:
+
+- External library internals (YamlDotNet, Microsoft.Extensions.FileSystemGlobbing,
+  DemaConsulting.TestResults)
+- Build pipeline configuration
+- Deployment and packaging
+
+## Software Structure
+
+The following tree shows how the FileAssert software items are organized across the system,
+subsystem, and unit levels:
+
+```text
+FileAssert (System)
+├── Program (Unit)
+├── Cli (Subsystem)
+│   └── Context (Unit)
+├── Configuration (Subsystem)
+│   ├── FileAssertConfig (Unit)
+│   └── FileAssertData (Unit)
+├── Modeling (Subsystem)
+│   ├── FileAssertTest (Unit)
+│   ├── FileAssertFile (Unit)
+│   └── FileAssertRule (Unit)
+├── Utilities (Subsystem)
+│   └── PathHelpers (Unit)
+└── SelfTest (Subsystem)
+    └── Validation (Unit)
+```
+
+Each unit is described in detail in its own chapter within this document.
+
+## Folder Layout
+
+The source code folder structure mirrors the top-level subsystem breakdown above, giving
+reviewers an explicit navigation aid from design to code:
+
+```text
+src/DemaConsulting.FileAssert/
+├── Program.cs                      — entry point and execution orchestrator
+├── Cli/
+│   └── Context.cs                  — command-line argument parser and I/O owner
+├── Configuration/
+│   ├── FileAssertConfig.cs         — top-level configuration loader and test runner
+│   └── FileAssertData.cs           — YAML data transfer objects
+├── Modeling/
+│   ├── FileAssertTest.cs           — named test with file assertions and tag filtering
+│   ├── FileAssertFile.cs           — glob pattern matcher with count constraints and rules
+│   └── FileAssertRule.cs           — abstract content validation rule hierarchy
+├── Utilities/
+│   └── PathHelpers.cs              — safe path-combination utility
+└── SelfTest/
+    └── Validation.cs               — self-validation test runner
+```
+
+The test project mirrors the same layout under `test/DemaConsulting.FileAssert.Tests/`.
+
+## Document Conventions
+
+Throughout this document:
+
+- Class names, method names, property names, and file names appear in `monospace` font.
+- The word **shall** denotes a design constraint that the implementation must satisfy.
+- Section headings within each unit chapter follow a consistent structure: overview, data model,
+  methods/algorithms, and interactions with other units.
+- Text tables are used in preference to diagrams, which may not render in all PDF viewers.
+
+## References
+
+- [FileAssert User Guide][guide]
+- [FileAssert Repository][repo]
+
+[guide]: ../../README.md
+[repo]: https://github.com/demaconsulting/FileAssert