diff --git a/README.md b/README.md
index 022249e2..4752ceed 100644
--- a/README.md
+++ b/README.md
@@ -158,6 +158,7 @@ In a similar way, not providing a WiFi configuration will force the target into
 
 - [SW Architecture](./doc/architecture/README.md)
 - [SW Configuration](./doc/configuration/README.md)
+- [Coding Guideline](./doc/CodingGuideline.md)
 
 ## Used Libraries
 
diff --git a/doc/CodingGuideline.md b/doc/CodingGuideline.md
new file mode 100644
index 00000000..dfc94080
--- /dev/null
+++ b/doc/CodingGuideline.md
@@ -0,0 +1,270 @@
+# Coding Guideline <!-- omit in toc -->
+
+Derived from existing source code in this repository.
+
+## Table of Contents <!-- omit in toc -->
+
+- [File Structure](#file-structure)
+- [Naming](#naming)
+- [Headers](#headers)
+- [Doxygen](#doxygen)
+- [Class Layout](#class-layout)
+- [Patterns](#patterns)
+  - [Singleton](#singleton)
+  - [State Machine](#state-machine)
+- [Formatting](#formatting)
+- [Conditionals](#conditionals)
+  - [Switch](#switch)
+- [Parameters](#parameters)
+- [Logging](#logging)
+- [Tests](#tests)
+
+## File Structure
+
+Every file starts with the full MIT license header, followed by a `DESCRIPTION` Doxygen block, then fixed section separators in this order:
+
+```cpp
+/* MIT License
+ *
+ * Copyright (c) 2023 - 2026 Andreas Merkle <web@blue-andi.de>
+ * ...
+ */
+
+/*******************************************************************************
+    DESCRIPTION
+*******************************************************************************/
+/**
+ * @file
+ * @brief  Short description.
+ * @author First Last <email@example.com>
+ *
+ * @addtogroup GroupName
+ *
+ * @{
+ */
+```
+
+Section separators inside `.cpp` files appear in this fixed order:
+
+```
+Includes / Compiler Switches / Macros / Types and classes /
+Prototypes / Local Variables / Public Methods / Protected Methods /
+Private Methods / External Functions / Local Functions
+```
+
+Each section uses a 80-character comment banner:
+
+```cpp
+/******************************************************************************
+ * Includes
+ *****************************************************************************/
+```
+
+Close every header file with:
+
+```cpp
+/** @} */
+```
+
+---
+
+## Naming
+
+| Element                          | Convention               | Example                                             |
+| -------------------------------- | ------------------------ | --------------------------------------------------- |
+| Class                            | PascalCase               | `StateMachine`, `StartupState`                      |
+| Interface / abstract base        | Prefix `I`               | `IState`, `IBoard`                                  |
+| Method                           | camelCase                | `setup()`, `getInstance()`                          |
+| Private member variable          | `m_` prefix + camelCase  | `m_isActive`, `m_duration`                          |
+| Constant / `static const` member | `UPPER_SNAKE_CASE`       | `MIN_BATTERY_LEVEL`, `SEND_WAYPOINT_TIMER_INTERVAL` |
+| File-scope static variable       | `g` prefix + camelCase   | `gLogSinkSerial`                                    |
+| Macro parameter                  | double-underscore prefix | `SIMPLE_TIMER_SECONDS(__timeInS)`                   |
+| Enum value                       | `UPPER_SNAKE_CASE`       | `CMD_GET_MAX_SPEED`, `CMD_NONE`                     |
+
+Use `uint8_t`, `uint16_t`, `uint32_t` etc. for sized integer types. Append `U` to unsigned integer literals (`50U`, `1000U`).
+
+---
+
+## Headers
+
+Use `#ifndef` include guards — not `#pragma once`:
+
+```cpp
+#ifndef STATE_MACHINE_H
+#define STATE_MACHINE_H
+// ...
+#endif /* STATE_MACHINE_H */
+```
+
+Template/header-only implementations use `.hpp`, all others use `.h` / `.cpp`.
+
+Include order (within a file):
+1. Own header (`"App.h"`)
+2. Project headers (`<Board.h>`, `<Logging.h>`)
+3. Third-party / Arduino headers (`<ArduinoJson.h>`)
+
+---
+
+## Doxygen
+
+Every public method, class, and file gets a Doxygen doc comment. Use `/** */` style.
+
+```cpp
+/**
+ * Set latest vehicle data received from RadonUlzer.
+ *
+ * @param[in] waypoint  Latest vehicle data.
+ */
+void setLatestVehicleData(const Waypoint& waypoint);
+
+/**
+ * Get pending command.
+ *
+ * @param[out] cmd  Pending command.
+ *
+ * @returns true if a command is pending, otherwise false.
+ */
+bool getPendingCommand(Command& cmd);
+```
+
+Use `@param[in]`, `@param[out]`, `@param[in,out]`, `@returns`. Document deleted copy operations too (mark as "Not allowed.").
+
+---
+
+## Class Layout
+
+Ordering within a class: `public` → `protected` → `private`. Group within each section:
+
+1. Constructors / destructor
+2. Static factory / `getInstance()`
+3. Public methods
+4. Static constants
+5. Member variables
+6. Deleted copy constructor / assignment operator
+
+---
+
+## Patterns
+
+### Singleton
+
+```cpp
+class StartupState : public IState
+{
+public:
+    static StartupState& getInstance()
+    {
+        static StartupState instance;
+        /* Singleton idiom to force initialization during first usage. */
+        return instance;
+    }
+
+private:
+    StartupState() : IState(), m_isActive(false) { }
+    virtual ~StartupState() { }
+    StartupState(const StartupState& state);             // Not allowed.
+    StartupState& operator=(const StartupState& state);  // Not allowed.
+};
+```
+
+### State Machine
+
+States implement `IState` with three lifecycle methods:
+
+```cpp
+class IState
+{
+public:
+    virtual void entry()                 = 0;
+    virtual void process(StateMachine& sm) = 0;
+    virtual void exit()                  = 0;
+};
+```
+
+Concrete states mark the overrides `final` and are themselves singletons.
+
+---
+
+## Formatting
+
+- **Indentation**: 4 spaces, no tabs.
+- **Braces**: Allman style — opening brace on its own line for classes and functions; on the same line for `if`/`else`/`for`/`while`.
+- **Constructor initializer lists**: one member per line, colon on the first line.
+
+```cpp
+App() :
+    m_channelId(0U),
+    m_timer(),
+    m_stateMachine()
+{
+}
+```
+
+- **Variable alignment**: align `=` signs in blocks of related declarations.
+
+```cpp
+bool             isSuccessful = false;
+SettingsHandler& settings     = SettingsHandler::getInstance();
+Board&           board        = Board::getInstance();
+```
+
+---
+
+## Conditionals
+
+Write comparisons in "Yoda style" with explicit `true`/`false`:
+
+```cpp
+if (true == m_isActive)   { ... }
+if (false == isSuccessful) { ... }
+if (nullptr != pData)     { ... }
+```
+
+Use `nullptr` (never `NULL` or `0`) for pointer checks.
+
+### Switch
+
+Case labels are at the same indentation level as `switch`. Mark intentional fall-through explicitly:
+
+```cpp
+switch (status)
+{
+case STATUS_A:
+    /* Fallthrough */
+case STATUS_B:
+    handleError();
+    break;
+
+default:
+    break;
+}
+```
+
+---
+
+## Parameters
+
+Pass non-trivial input arguments as `const` reference; output arguments as non-const reference:
+
+```cpp
+void process(const Waypoint& in, Result& out);
+```
+
+---
+
+## Logging
+
+Use severity macros — no direct `printf` in application code:
+
+```cpp
+LOG_FATAL("Setup failed.");
+LOG_ERROR("Unexpected state %d.", state);
+LOG_WARNING("Battery low.");
+LOG_DEBUG("Speed: %d mm/s.", speed);
+```
+
+---
+
+## Tests
+
+Unit tests use the Unity framework (`TEST_ASSERT_*`, `RUN_TEST`). Each suite has `setUp()` and `tearDown()` functions. Test files live under `test/test_<Name>/`.