MMTk (Memory Management Toolkit) is a high-performance, portable garbage collection framework written in Rust. It provides a library of reusable GC algorithms (plans) including generational, concurrent, mark-sweep, immix, reference-counting (LXR), and mark-compact variants. The system exposes a trait-based VM binding interface (VMBinding) that language runtimes implement to integrate MMTk as their garbage collector. The architecture is modular: a parallel work-stealing scheduler dispatches GC work packets, policy spaces encapsulate memory allocation strategies (bump, free-list, immix, large object, etc.), and a flexible metadata system supports both header-resident and side metadata. The crate is not a standalone application; it is consumed as an rlib by VM bindings such as OpenJDK, JikesRVM, V8, Julia, and Ruby. Extensive use of procedural macros (HasSpaces, PlanTraceObject) reduces boilerplate for adding new GC plans. The codebase includes performance tracing tools (eBPF, Perfetto) and a comprehensive test suite with mock VM support.