The modern study of fertility effectively begins just half a century ago with the founding of the Population Investigation Committee in the mid1930s; this half century has presented the most difficult problems of explaining fertility trends since reliable birth records began to be kept. The fluctuating trend from 1936 to 1986 followed half a century of continuous decline; in 1986 with more powerful methods of analysis at our disposal, and more elaborate data, we can have no more confidence in our speculations about trends in the next two or three decades than would have been justified in the mid1930s. There are however some approximate parallels between the fertility patterns of the mid1930s and those of the mid1980s which may serve as a point of entry into a very complex field of discussion. By 1938 it seemed clear that the birth total for 1933 marked an important halt in, if not the actual terminus of, the long decline in fertility from the 1880s, unbroken save for the brief post war upturn of 1920/21. In 1986 it seems equally clear that the indices for 1976/77 marked a change in trend, not necessarily towards more increase, but at least a check in the significant decline in fertility which began in the mid-1960s. However, there is no true comparison possible between the long sustained trend from 1885 to 1930, and the short episode after about 1966. In the later 1930s there was some concern about the possibility of actual population decline, with apparently alarming probable consequences. In the 1980s, with more sophisticated information at our disposal and with apparently much less general fear of decline, concern is centred on specific problems, such as the certainty of increased old age dependency, not itself a fertility problem in other than an historical sense, but indirectly raising the question of future fertility prospects. Between these periods lies the key aspect of recent uncertainties, the curious short term (but widespread and very significant) upward trend of the late 1950s and early 1960s, still unexplained in cause, although extensively measured and described, and influencing demographic prospects well into the next century. This half-century has been one of quite dramatic social change. It embraces the depression of the 1930s, the growing fear of war, and then its actuality, post war reconstruction and recovery with increasing economic and social optimism in the 1960s followed by the recession after 1973, and the reluctant appreciation of the weakening economic and political position of the United Kingdom. There has also been a background of world tension, dominated by the presence of nuclear weapons. There have been important changes in the status of women, adolescents and young people; in the initiation and development of the welfare state; and significantly in an 'egalitarianism' of claims to increased personal consumption as of right. Not least, given the focus of this Chapter, has been the complete diffusion and acceptance of easily available and technically efficient contraception; abortion and sterilisation have followed as recognised elements in public and personal health provision. Above all there has been