The work that this book represents is something I have wanted to do since 1979. While in Ireland, probably in 2001, I sketched some parts of a small operating system speci?cation in Z but left it because of other duties. In 2002, I worked on the sketches again but was interrupted. Finally, in April, 2005, I decided to devote some time to it and produced what amounted to a ?rst version of the kernel to be found in Chapter 3 of this book. I even produced a few proofs, just to show that I was not on a completely insane tack. I decided to suggest the material as the subject of a book to Beverley Ford. The material was sent on a Thursday (I think). The following Monday, I received an email from her saying that it had gone out for review. The review process took less than 2 weeks; the response was as surprising as it was encouraging: a de?nite acceptance. So I got on with it. This book is intended as a new way to approach operating systems - sign in general, and kernel design in particular. It was partly driven by the old ambition mentioned above, by the need for greater clarity where it comes to kernels and by the need, as I see it, for a better foundation for operating systemsdesign.Securityaspects,too,playedapart—asnotedintheintrod- tory chapter, if a system’s kernel is insecure or unreliable, it will undermine attemptstoconstructsecuresoftwareontopofit.Securitydoesnototherwise play a part in this book.