L4: a growing family of 2nd-generation microkernel realtime operating systems (RTOSs) that are fast, small, lean, maintainable, readable, preemptible, support hard priorities, use non-blocking synchronization for kernel objects to guarantee priority inheritance and ensure runnable high-priority processes never block waiting for lower ones. All scale well from tiny embedded systems to huge servers, and use hierarchical external pagers and guarded page tables, with fast, message-based, synchronous IPC, simply-used external paging mechanisms, security mechanism based on secure domains: tasks, clans, chiefs. Kernels try to implement only a minimal set of abstractions on which OSs can be built flexibly. L3 has system-wide persistence. All kernels are roughly compatible and many run on many platforms: ARM; MIPS; 68k, PPC; x86; L4Linux ports exist. Goals: create, prove, establish new methodology for systems that helps manage growing OS complexity and minimizes legacy dependence. [Open Source, GPL]