“This document is intended as an up-to-date description on the fundamental concepts related to the anykernel and rump kernels. It is based on the dissertation written in 2011 and early 2012: Flexible Operating System Internals: The Design and Implementation of the Anykernel and Rump Kernels. The major change with rump kernels since the first edition is a shift in focus and motivation. In work leading up to the first edition, rump kernels were about running kernel components in userspace. That work defined the core architecture, and that definition is still valid and accurate. Since then, work has focused on harnessing the potential of rump kernels for building entirely new computation stacks. Since this edition of the book is no longer an academic document, we do not support every statement we make with a citation or experiment. In fact, we also take the liberty to present opinions which are open for debate.”