L4 is a family of second-generation microkernels, generally used to implement Unix-like operating systems, but also used in a variety of other systems.
L4, like its predecessor L3 microkernel, was created by German computer scientist Jochen Liedtke as a response to the poor performance of earlier microkernel-based operating systems. Liedtke felt that a system designed from the start for high performance, rather than other goals, could produce a microkernel of practical use. His original implementation in hand-coded Intel i386-specific assembly language code in 1993 sparked intense interest in the computer industry. Since its introduction, L4 has been developed for platform independence and also in improving security, isolation, and robustness.
There have been various re-implementations of the original binary L4 kernel interface (ABI) and its successors, including L4Ka::Pistachio (Uni Karlsruhe), L4/MIPS (UNSW), Fiasco (WrmLab). For this reason, the name L4 has been generalized and no longer only refers to Liedtke's original implementation. It now applies to the whole microkernel family including the L4 kernel interface and its different versions.
L4 is widely deployed. One variant, OKL4 from Open Kernel Labs, shipped in billions of mobile devices.