Although the birth-rate has declined in all Western nations since the middle of the nineteenth century, the decline has not been equal among the various groups that compose their population. Measures of fertility for national units conceal differences in the fertility of the many distinct groups in urban-industrial societies. Changes in the pattern of these differences are of particular significance in the later stages of the demographic transition from high to low birth-rates and death-rates which the Western world has undergone in the past century. Changes in the birth-rate rather than in the death-rate are the main determinants of growth in societies where mortality has been brought under control by modern medicine and public health practices.Demographers have often treated differential fertility as a special, virtually autonomous subject instead of viewing it in the broad historical context of the Western demographic transition. In fact, as J. W. Innes has pointed out, few studies have been made of trends in differential fertility by comparison with the numerous studies which simply establish the existence of group differences in fertility at a single point in time. The present paper is concerned with class differences—probably the most pervasive of all group differences in advanced societies. The attempt is made to gather together by historical period the available data on trends in class differences in fertility for several Western countries in order to present a systematic picture of the way in which these differences have evolved in modern times.The problem of dividing a population into socio-economic classes that are genuinely distinct from one another in a sociologically meaningful sense has been widely discussed by sociologists. Demographers, however, usually work with official data which provide only limited information on the characteristics of populations. They are, therefore, unable to employ the more refined indices of class developed by sociologists and are forced to use as indices such relatively simple objective attributes as income, occupation, or education.