This book describes the NetBSD Operating System internals. The main idea behind it is to provide solid documentation for contributors that wish to develop extensions for NetBSD or want to improve its existing code. Ideally, there should be no need to reverse-engineer the system's code in order to understand how something works. A lot of work is still required to finish this book: some chapters are not finished and some are not even started. Those parts that are planned but which are still pending to do are already part of the book but are clearly marked as incomplete by using a XXX marker. This book is currently maintained by the NetBSD www team (www@NetBSD.org). Corrections, suggestions and extensions should be sent to that address.