Principles and concepts of development are reviewed in relation to life-span issues noting the need to consider: development in its social context; timing of experiences; intrinsic and experiential factors; continuities and discontinuities; parallels and differences between normal and abnormal development; heterotypic and homotypic continuities; key life transitions; risk and protective factors; indirect chain affects; mediating mechanisms; age as an index of maturational and experiential factors. Developmental findings from childhood to adult longitudinal studies are reviewed for possible mediating factors. These include: genetic mechanisms; the (non-genetic) biological substrate; shaping of the environment; cognitive and social skills; self-esteem and self-efficacy; habits, cognitive sets and coping styles; links between experiences.