The existence of so many strangely puzzling, even contradictory, aspects of 'time' is due, I think, to the fact that we obtain our ideas about temporal succession from more than one source - from inner experience, on the one side, and from the physical world on the other. 'Time' is thus a composite notion and as soon as we distinguish clearly between the ideas deriving from the different sources it becomes apparent that there is not just one time-concept but several. Perhaps they should be called variants, but in any case they need to be seen as distinct. In this book I shall aim at characteri­ sing what I believe to be the three most basic of them. These form a sort of hierarchy of increasing richness, but diminishing symmetry. Any adequate inquiry into 'time' is necessarily partly scientific and partly philosophical. This creates a difficulty since what may be elementary reading to scientists may not be so to philosophers, and vice versa. For this reason I have sought to present the book at a level which is less 'advanced' than that of a specialist monograph. Due to my own background there is an inevitable bias towards the scientific aspects oftime. Certainly the issues I have taken up are very diffe­ rent from those discussed in several recent books on the subject by philoso­ phers.