E. B. Wilson stated in the preface to the third edition of The Cell in Development and Heredity: 'Every writer must treat the subject from the standpoint given by those fields of wnrk in which he is most at home; and at best he can only try to indicate a few of the points of contact between those fields and others'. The aim of this book is to provide an overview of structural and functional aspects of fertiliz­ ation processes in a manner that would be helpful not only to specialists in the field but also to investigators in related disciplines and to advanced undergraduate and graduate students in the bio­ logical sciences. Fundamental descriptive accounts at the light and electron microscopic levels of observation have been combined with analytical studies - physiological and biochemical investigations of fertilization in invertebrates and vertebrates. A comparative approach to fertilization is presented and, although a variety of animals are referred to, additional space is purposely given to organisms that have been, and continue to be, popular research material. The text does not pretend to be comprehensive and admittedly does not cover all aspects of the field or areas related to the general subject. Historical reviews and technical details are presented only to the extent necessary to formulate an orientation to and a perspective on individual topics.