Secondary data analysis of large longitudinal and national data sets is a standard method used in many social sciences to answer complex questions regarding behavior. In this article, we detail the advantages of using these data sets to study developmental questions across the life span. First, we provide an overview of how using secondary data can increase studies' scientific integrity. Then, we detail where and how data sets can be obtained that answer specific questions. Finally, we discuss methodological issues related to using longitudinal, population data sets. These data sets can enhance science and test theories by increasing the rigor and generalizability of research to the general population, making secondary data analysis an important method to consider.