Acknowledgements
I am grateful to Thibault Damour, Institut des Hautes Études Scientifiques, Paris and member of the Academy of Sciences, Paris, for his helpful comments to Part I of this review, and for his advice in the final stages of writing Part II. My thanks go also to a referee whose thorough and thoughtful comments have helped to improve this article. It is my great pleasure to thank the staff of Living Reviews in Relativity, notably Mrs. Osswald and Mr. Schulz, for their friendly, generous and careful assistance.