Description:This book is a sequel to Time and Modality. Many problems raised in the latter have now been solved, and new ones have been raised in their turn, and I have tried to record some of these developments, and to carry on with some further ones. I have also become aware of the continuing importance of some earlier writings, including some of my own, which I was formerly inclined to think had been simply superseded; so I have something to say about those too. But I have tried to make the book self-contained, presupposing nothing but a few facts, mostly about the better-known systems of modal logic, which can easily be found in the literature.