Existing data sets can be an efficient, powerful, and readily available resource for addressing questions about developmental science. Many of the available databases contain hundreds of variables of interest to developmental psychologists, track participants longitudinally, and have representative samples. In this article, the authors discuss the benefits and limitations of secondary data analysis so that researchers can make informed decisions about when it makes sense to use existing data and when it does not. They then provide an overview of best practices for handling the most common conceptual, methodological, and practical issues that arise when adopting this approach. (Contains 2 tables.)