Although there are a number of excellent current reviews on one or another aspect of cytomegalovirus, the last comprehensive treatment of this subject was that of Krech et al. (1971a). In view of the amazing advances in the virological, epidemiologic, and clinical knowledge of cytomegaloviruses, an up-to-date book is needed. Such a work should cover many areas of expertise and a voluminous technical literature. Each area might have been reviewed and analyzed by workers more expert than myself. However, I have embarked on the entire venture alone in order to attain unity and continuity in this book, characteristics that are not easily achieved in the more popular multiauthored works. I have tried to review the literature and to provide a critical summary for each area discussed. To do this, I provide as much of the primary data of the relevant works as needed and not just the qualitative conclusions. Inevitably, the flow of the narrative may be interrupted by dry facts and figures. However, such information is essential to make this a meaningful reference work. But for those not interested in such details, I have provided at what I hope are crucial points critiques and summaries. This book is not an exhaustive review of all the literature. This is probably no longer possible or even desirable. By selection, however, one runs the risk of having missed or ignored important papers. I am keenly aware of this, and I wish to apologize for such oversight, if that is possible.