This national longitudinal study follows the first 5 years of life of 13,135 children in England, Scotland, and Wales who were born during a single week in April 1970. Part One examines the concept and extent of social inequality in the sample of children and compares the social circumstances of this third cohort with those of the earlier two studies. Discussion then focuses on family disruption and describes how loss of a parent can result in downward social mobility. The division of labor in the home and the extent to which fathers have taken part in domestic and child care activities is analyzed. Social and family factors are compared with regard to their effects on children's cognitive development and on behavioral adjustment and depression in mothers. Part Two discusses the pattern of entry to infant schools in Britain and describes the relationship of such entry to use of preschool educational services. Different social and family factors influencing the type of preschool experiences acquired and the extent of mothers' involvement are revealed. Additionally, the association between preschool attendance, and cognitive ability and behavioral adjustment is examined. Part Three considers factors influencing mothers' paid employment and examines the extent and nature of such work. The attitudes of employed and nonemployed mothers toward maternal employment are contrasted, and reasons for mothers' working are reported. Child care arrangements made by working mothers are described, and an analysis is made of the association of maternal employment with children's behavior and ability and with maternal depression. Part Four describes how the survey was carried out, traces rates and possible sources of bias due to nonresponse, and provides detailed explanations of assessments and methods of analysis. (RH)