Fody is a .NET build-time IL weaving engine that integrates with MSBuild. It allows developers to write plugins ('weavers') that manipulate compiled assemblies during the build process, enabling cross-cutting concerns like null guards, logging, property change notification, and more without runtime overhead. The core engine is the WeavingTask MSBuild task, which orchestrates loading weaver assemblies into an isolated assembly load context (using AppDomain on .NET Framework, AssemblyLoadContext on .NET Core), executing them against the target module via Mono.Cecil, and writing the modified assembly. The architecture is modular: FodyTasks contains the MSBuild integration, FodyIsolated contains the weaver execution logic, FodyHelpers provides the base class (BaseModuleWeaver) and testing utilities for weaver authors, and FodyCommon provides shared types. Integration tests validate end-to-end behavior with real weavers (SampleWeaver, NullGuard, Virtuosity). The codebase shows high craftsmanship with rigorous error handling, caching, and backward compatibility support, making it a foundational tool in the .NET AOP ecosystem.