This short paper provides some guidelines to help researchers in child and adolescent development procure the racial/ethnic and socioeconomic information that will best determine how to assign youngsters to ethnic or SES groups. These guidelines are necessarily general. They will need to be adapted thoughtfully by each investigator because, as is generally true, how to define a measure depends intimately on the nature of the research problem. In preparing these guidelines, we have taken into account current practice at the Bureau of the Census, research traditions developed by sociologists who have mainly been concerned with adults, and challenges posed by the changing character of the U.S. population and its family forms. We are extremely grateful to the many social scientists listed below who have contributed so generously to our thinking, but especially to Robert Hauser. Naturally, any errors or opacities that remain are our responsibility.