This book deals with the relationship between gravitation and elementary particle physics, and the implications of these subjects for astrophysics. There has, in recent years, been renewed interest in theories that connect up gravitation and particle physics, and in the astrophysical consequences of such theories. Some of these accounts involve a time-variation of the Newtonian gravitational parameter, G. In this respect, the present book may be regarded as a companion to my Cosmology and Geophysics (Hilger, Bristol, 1978). There is some overlap as regards the discussion of G-variability, but the emphasis in the present book is on astrophysics while the emphasis in the other one is on geophysics. The subject is a very broad one indeed, and in giving a review of it I have adopted a somewhat unorthodox way of presenting the material involved. The main reason for this is that a review of such a wide subject should aim at two levels: the level of the person who is interested in it, and the level of the person who is professionally engaged in research into it. To achieve such a two-level coverage, I have split the text up into two parts. The first part (Chapters 1-7) represents a relatively non-technical overview of the subject, while the second part (Chapters 8-11) represents a technical examination of the most important aspects of non-Einsteinian gravitational theory and its relation to astrophysics.