diff --git a/README.md b/README.md
index e310270..7bb0a8f 100644
--- a/README.md
+++ b/README.md
@@ -1,12 +1,60 @@
 # balloon-obc-401
 A flexible on-board-computer for high altitude balloons based on the dual core STM32H7 series running FreeRTOS.
 
-## Setup / Build Instructions
-1. Windows -> ARM cross toolchain available [here](https://developer.arm.com/downloads/-/arm-gnu-toolchain-downloads).
-2. Run `git submodule update --init --remote --recursive`
-3. `cmake -DCMAKE_C_COMPILER=... -DCMAKE_CXX_COMPILER=... -Bbuild .`
-4. `cmake --build build --config Debug --target all`
-5. Profit
+## Getting Started
+### Dependencies
+Before starting you require the following:
+- [ARM Toolchain](https://developer.arm.com/downloads/-/arm-gnu-toolchain-downloads)
+- [CMake](https://cmake.org/download/)
+- [Clang](https://releases.llvm.org/download.html)
+- [STLINK](https://github.com/stlink-org/stlink)
+- [STM32CubeMX](https://www.st.com/content/st_com/en/stm32cubemx.html)
+
+```sh
+# Arch Installation
+sudo pacman -Syu gcc-arm-none-eabi gcc-arm-none-eabi-newlib clang cmake stlink
+paru -S stm32cubemx
+```
+
+This repository uses Git submodules for software libraries. The submodules can be downloaded with:
+```sh
+git submodule update --init --remote --recursive
+```
+
+### Building
+All commands should be run from the root directory of the project. To build everything run:
+```sh
+cmake -DCMAKE_C_COMPILER=[C_COMPILER] -DCMAKE_CXX_COMPILER=[CPP_COMPILER] -Bbuild .
+cmake --build build --config Debug --target all
+```
+`[C_COMPILER]` and `[CPP_COMPILER]` must be replaced with with the compiler which corresponds to the hardware being targetted. For local testing this is `gcc` and `g++`, for deployment to STM32 this is `gcc-arm-none-gcc` and `gcc-arm-none-g++`.
+
+### Local Testing
+Units tests are run with GTest. To run all tests use CTest (a part of CMake).
+```sh
+cd build
+ctest
+```
+
+To debug a test which is failing with GDB the following procedure can be used.
+```sh
+# Find the command used to run the test to debug, replace [NAME_OF_TEST] with the name or a regex with matches it
+ctest -R $[NAME_OF_TEST] -V -N
+# Run GDB with the test command eg. gdb obc_m7  "-v" "test0"
+gdb [TEST_COMMAND]
+```
+
+### Device Testing
+Debugging is done with GDB.
+```sh
+# In a secondary terminal
+st-util
+
+# In the primary terminal
+arm-none-eabi-gdb build/obc_m7.elf
+target extended localhost:4242
+load
+```
 
 ## Multithreading
 Given the low level nature of embedded development, we do not have the luxury of symmetric multiprocessing.
diff --git a/common/Inc/obc/bus.hpp b/common/Inc/obc/bus.hpp
index fe19f8c..0bf993b 100644
--- a/common/Inc/obc/bus.hpp
+++ b/common/Inc/obc/bus.hpp
@@ -24,10 +24,12 @@
 #include <cassert>
 #include <concepts>
 #include <cstddef>
+#include <expected>
 #include <optional>
 #include <span>
 
 #include "obc/ipc/callback.hpp"
+#include "obc/utils/error.hpp"
 #include "obc/utils/handle.hpp"
 
 /**
@@ -157,9 +159,14 @@ template<typename T, typename M = BasicMessage>
 concept ListenBus =
     requires(T& bus, ipc::Callback<void, const M&>::DummyProvider&& cb) {
         typename T::ListenHandle;
+        typename T::ListenError;
 
-        { bus.Listen(cb.Func) } -> std::same_as<typename T::ListenHandle>;
-    } && std::semiregular<typename T::ListenHandle> && Message<M>;
+        {
+            bus.Listen(cb.Func)
+        } -> std::same_as<
+            std::expected<typename T::ListenHandle, typename T::ListenError>>;
+    } && std::semiregular<typename T::ListenHandle> &&
+    utils::MaybeError<typename T::ListenError> && Message<M>;
 
 /**
  * @brief A helper class for implementing listening functionality.
@@ -177,6 +184,7 @@ class ListenBusMixin {
 
   public:
     using ListenHandle = utils::Handle<ListenCallback>;
+    using ListenError  = utils::Never;
 
     /**
      * @brief Adds a listener to be notified upon receiving a message.
@@ -185,7 +193,7 @@ class ListenBusMixin {
      * @return An opaque handle that must be retained for the listener to remain
      * active.
      */
-    ListenHandle Listen(ListenCallback&& cb) {
+    std::expected<ListenHandle, ListenError> Listen(ListenCallback&& cb) {
         return ListenHandle(m_listeners, std::move(cb));
     }
 
@@ -226,12 +234,15 @@ concept RequestBus =
         ipc::Callback<void, const Res&>::DummyProvider cb, Buf& buf
     ) {
         typename T::RequestHandle;
+        typename T::RequestError;
 
         {
             bus.Request(req, cb.Func, buf)
-        } -> std::same_as<typename T::RequestHandle>;
+        } -> std::same_as<
+            std::expected<typename T::RequestHandle, typename T::RequestError>>;
     } &&
-    std::semiregular<typename T::RequestHandle> && Message<Req> &&
+    std::semiregular<typename T::RequestHandle> &&
+    utils::MaybeError<typename T::RequestError> && Message<Req> &&
     Message<Res> && Buffer<Buf, BufUnit>;
 
 /**
@@ -261,7 +272,7 @@ template<
     typename T, typename Req = BasicMessage, typename Res = BasicMessage,
     typename Flt = ipc::Callback<bool, const Res&>>
 concept RequestBusMixinRequirements = requires(T& bus, const Req& req) {
-    { bus.IssueRequest(req) } -> std::same_as<Flt>;
+    { bus.IssueRequest(req) } -> utils::ExpectedReturn<Flt>;
 } && Message<Req> && Message<Res> && MessageFilter<Flt>;
 
 /**
@@ -290,6 +301,9 @@ class RequestBusMixin {
 
   public:
     using RequestHandle = utils::Handle<RequestHandleData>;
+    using RequestError  = decltype(Derived().IssueRequest(
+        std::declval<const Req&>(), std::declval<const Res&>()
+    ))::error_type;
 
     /**
      * @brief Sends a request on the bus.
@@ -299,14 +313,16 @@ class RequestBusMixin {
      * @return An opaque handle that must be retained until the request is
      * fulfilled or the response is no longer desired.
      */
-    RequestHandle Request(const Req& req, RequestCallback&& cb, Buf buf) {
-        return RequestHandle(
-            m_request_handlers,
-            {
-                .filter   = Derived().IssueRequest(req, buf),
-                .callback = std::move(cb),
-            }
-        );
+    std::expected<RequestHandle, RequestError> Request(
+        const Req& req, RequestCallback&& cb, Buf buf
+    ) {
+        if (auto filter = Derived().IssueRequest(req, buf)) {
+            return RequestHandle(
+                m_request_handlers, {.filter = filter.value(), .callback = cb}
+            );
+        } else {
+            return std::unexpected(filter.error());
+        }
     }
 
   protected:
@@ -351,10 +367,14 @@ template<typename T, typename Req = BasicMessage, typename Res = BasicMessage>
 concept ProcessBus =
     requires(T& bus, ipc::Callback<std::optional<Res>, Req>::DummyProvider cb) {
         typename T::ProcessHandle;
+        typename T::ProcessError;
 
-        { bus.Process(cb.Func) } -> std::same_as<typename T::ProcessHandle>;
-    } && std::semiregular<typename T::ProcessHandle> && Message<Req> &&
-    Message<Res>;
+        {
+            bus.Process(cb.Func)
+        } -> std::same_as<
+            std::expected<typename T::ProcessHandle, typename T::ProcessError>>;
+    } && std::semiregular<typename T::ProcessHandle> &&
+    utils::MaybeError<typename T::ProcessError> && Message<Req> && Message<Res>;
 
 /**
  * @brief Specifies the required methods for the class \ref ProcessBusMixin is
@@ -385,6 +405,7 @@ class ProcessBusMixin {
 
   public:
     using ProcessHandle = utils::Handle<ProcessCallback>;
+    using ProcessError  = utils::Never;
 
     /**
      * @brief Registers a processor callback.
@@ -394,7 +415,7 @@ class ProcessBusMixin {
      * @return An opaque handle that must be retained for the processor to
      * remain active.
      */
-    ProcessHandle Process(ProcessCallback&& cb) {
+    std::expected<ProcessHandle, ProcessError> Process(ProcessCallback&& cb) {
         return ProcessHandle(m_processors, std::move(cb));
     }
 
diff --git a/common/Inc/obc/port.hpp b/common/Inc/obc/port.hpp
index 24e3201..2c847e1 100644
--- a/common/Inc/obc/port.hpp
+++ b/common/Inc/obc/port.hpp
@@ -22,6 +22,8 @@
 
 #include <iterator>
 
+#include "utils/error.hpp"
+
 /**
  * @brief Defines a common interface for IO ports.
  *
@@ -33,6 +35,12 @@
  * used to signal intent, however there may be additional constraints placed on
  * them in future.
  *
+ * Ports should be able to source or sink an unlimited amount of data. Reaching
+ * the end of the iterator signals some sort of error state. The error can be
+ * queried from the iterator.
+ *
+ * @todo Timeouts and error states
+ *
  * @warning Data is only pulled/pushed to the port upon calling the increment
  * operator, this operation may block for an unspecified duration.
  *
diff --git a/common/Inc/obc/utils/error.hpp b/common/Inc/obc/utils/error.hpp
new file mode 100644
index 0000000..34d0718
--- /dev/null
+++ b/common/Inc/obc/utils/error.hpp
@@ -0,0 +1,37 @@
+#include <concepts>
+#include <expected>
+
+#include "utils/meta.hpp"
+
+namespace obc::utils {
+/**
+ * @brief A class which implements the error interface or
+ * is the special Never type (ie. not an error).
+ *
+ * @todo Implment the error interface, currently only `Never`
+ * is valid.
+ *
+ * @warning MaybeError is not a valid return type for a function
+ * which may fail or return nothing. You probably want to use
+ * `std::optional<Error>` instead.
+ */
+template<typename T>
+concept MaybeError = std::same_as<T, Never>;
+
+template<typename T, typename R>
+concept ExpectedReturn =
+    Specializes<T, std::expected> && MaybeError<typename T::error_type> &&
+    std::convertible_to<T, std::expected<R, typename T::error_type>>;
+
+#define UNWRAP_ASSIGN_IMPL(lhs, rexpr, temp)                                 \
+    auto temp = rexpr;                                                  \
+    static_assert(obc::utils::ExpectedReturn<std::remove_cvref<temp>>); \
+    if constexpr (!std::same_as<                                        \
+                      std::remove_cvref<temp>::error_type,              \
+                      obc::utils::Never>) {                             \
+        if (!temp) return std::unexpected(std::move(temp).error());     \
+    }                                                                   \
+    lhs = std::move(temp.value());
+
+#define UNWRAP_ASSIGN(lhs, rexpr) UNWRAP_ASSIGN_IMPL(lhs, rexpr, TEMP_NAME)
+}  // namespace obc::utils
diff --git a/common/Inc/obc/utils/meta.hpp b/common/Inc/obc/utils/meta.hpp
index 67987b2..9e84170 100644
--- a/common/Inc/obc/utils/meta.hpp
+++ b/common/Inc/obc/utils/meta.hpp
@@ -29,6 +29,26 @@ concept OptionLike = requires(T t) {
     { *t } -> std::convertible_to<U>;
 };
 
+/**
+ * @brief An appoximation of a bottom type.
+ *
+ * Bottom types are used to represent values which cannot exist in contexts
+ * where a type must be specified. In other words, a type for which there
+ * are no valid values.
+ *
+ * Unlike a true bottom type (like those in Rust), this cannot be cast into
+ * any other type. This was an intentional design choice to prevent the
+ * accidental creation and
+ *
+ * This type can be privately subclassed by other marker types to prevent
+ * construction and use.
+ */
+struct Never {
+    Never()                        = delete;
+    Never(const Never&)            = delete;
+    Never& operator=(const Never&) = delete;
+};
+
 /**
  * @brief Check if a predicate is true for all elements of the zip of the
  * element types of tuples.
@@ -68,4 +88,19 @@ concept AllPairsConvertable = MultiwayConjunctionV<std::is_convertible, F, T>;
  */
 template<typename... Ts>
 class TypeAmalgam : public Ts... {};
+
+template<typename T, template<typename...> class Z>
+struct is_specialization_of : std::false_type {};
+
+template<typename... Args, template<typename...> class Z>
+struct is_specialization_of<Z<Args...>, Z> : std::true_type {};
+
+template<typename T, template<typename...> class Z>
+inline constexpr bool is_specialization_of_v =
+    is_specialization_of<T, Z>::value;
+
+template<typename T, template<typename...> class Z>
+concept Specializes = is_specialization_of_v<T, Z>;
+
+#define TEMP_NAME temp_##__COUNTER__
 }  // namespace obc::utils