Longitudinal panel data provide a unique opportunity to examine patterns and sources of economic and demographic change at the individual and family level.  These data are relevant to a host of policy issues, from the assessment of welfare program participation to an understanding of patterns of health care usage or of the determinants of retirement. Many policy issues require some understanding of the factors that lead up to a particular event, or of the consequences that stem from it.  Without repeated observations of the individuals concerned, however, such factors and consequences can only be inferred. Thus, our increasing store of longitudinal panel data holds the potential for major breakthroughs in our understanding of the basic determinants of economic and demographic change as they affect individuals and families over time.