This book is based on two longitudinal studies of behavior development, both conducted in Finland, a living laboratory setting for longitudinal research. Much of the book reports results from a longitudinal study begun in Jyvaskyla, Finland, in 1968, when its participants were school children. This longitudinal study is complemented by two Finnish twin-family studies, with parallel measures and overlapping aims, to yield insights into genetic and environmental sources of variation in early development and later outcomes. An array of findings from the two sets of longitudinal studies are presented, set within a theoretical framework of socioemotional development, and focused on both individual and familial predictors of health-related outcomes from childhood to early adulthood. Many contributors to this edited volume represent a second-tier of Finnish-USA collaborators. They analyzed data from the longitudinal studies as part of their advanced training, and their contributions to the book report recent results of such analyses.