Originally, L4 is the name of a second-generation microkernel designed and implemented by Jochen Liedtke, running on Intel 486 and Pentium CPUs. However, there are now numerous implementations of the L4 API on several hardware architectures.

Kernels based on the L4 API are second-generation microkernels. They are very lean and feature fast, message-based, synchronous IPC, simple-to-use external paging mechanisms, and a security mechanism based on secure domains (tasks, clans and chiefs). The kernels try to implement only a minimal set of abstractions on which operating systems can be built flexibly.