This book examines the types of secondary data sets available to researchers and how these have been used in the past and may be used in the future. While thousands of secondary data sets are available from private sources, universities, federal and other government agencies, and other public sources, I focus on data sets that are often used by social work and other social science researchers. Many of these data sets cover a wide variety of topics; others focus on particular topics or populations. Some are longitudinal, while others are cross-sectional, and they can cover either short or long periods of time. Some data sets use monthly information;others use annual or biannual information. Some data sets are nationally representative; others cover only specifi c populations. I examine the costs and benefi ts of using these different types of data sets and the reasons for using particular data sets given particular types of research questions or populations