[Keys to demographic changes in the new millennium].

This article describes the main features of the dynamics of demographic change in the population of Spain during the twentieth century, emphasizing the most recent developments: population ageing, the establishment of a very low fertility regime and the increasing importance of foreign immigration as well as the impact which all these have on the composition of households. Those aspects most directly related to the provision of health and social services is discussed, such as the impact of increased life expectancy on the incidence of illness and handicap in the population, the role of foreign immigration in providing personal services within the family and the health implications of the new model of delayed fertility. Finally, the health implications of population projections for the next 50 years are discussed.