Exokernel，一种操作系统内核架构，是一种极简化的微核心，约在1994年提出，由麻省理工学院平行与分散操作系统工作小组（Parallel and Distributed Operating Systems group）所发展。以这个概念而实作的其他操作系统内核，也被称为外核心（Exokernel），外内核、外核、或极限核心。

在Exokernel中，运作在核心空间的唯一行程就是内核，唯一工作就是负责分配系统资源，并防止用户行程访问到其他行程的资源。每个用户行程都拥有一个虚拟机，可以运行自己的操作系统。

Exokernel is an operating system kernel developed by the MIT Parallel and Distributed Operating Systems group, and also a class of similar operating systems.

Operating System Exokerns generally present the hardware-resources to applications through high level abstractions such as the (virtual) file-system. The idea behind exokernels is to force as few abstractions as possible on developers, enabling them to make as many decisions as possible about hardware abstractions. Exokernels are tiny, since functionality is limited to ensuring protection and multiplexing of resources, which are vastly simpler than conventional microkernels' implementation of message passing and monolithic kernels' implementation of abstractions.