Demography provides an important backcloth to the developments taking place in the UK, but it is neither unchanging in its nature nor neutral in its effects. The term is used here to refer to the size, distribution and composition of the population and the patterns of life-course events which maintain or alter these, notably the three basic components of change in population numbers (births, deaths and migration), but also the many factors that affect household formation and family building. The demography of the UK has undergone some major changes over the past three decades, including greater longevity, lower fertility, higher divorce rate, accelerating cohabitation, later marriage and childbearing, increasing lone parenthood, larger numbers living alone, a fluctuating exodus from large cities, a switch from net emigration to substantial international migration gain and the rise of the UK’s non-white population. Each of these raises policy issues for society, most of them immediate and obvious in their implications but some with major long-term impacts such as the continuing passage of the 1960s/1970s baby boom and bust through the age structure. The significance of these trends is evident in many other parts of this book, including changes in labour supply (Chapter 8), increasing ethnic diversity of urban areas (Chapters 10, 14), growing pressures on the countryside (Chapter 11), altered lifestyles and patterns of consumption (Chapters 12, 13) and the greying of the electorate (Chapter 16). At the same time, a wide range of factors influences the course of population change itself, prompting demographers to stress that their attempts at looking into the future are essentially projections and not predictions. Some examples of uncertainty will become apparent in highlighting developments since Compton’s (1991) review for a previous edition of this book.