This paper is a revision of one presented at the 61st Annual Meetings of the American Sociological Association in Miami Beach, Florida, August 29-September 1, 1966. It presents a systematic review and evaluation of the kinds of devices and resources that can be used to retrieve mobile cases in longitudinal and panel studies for further research. A number of alternatives are considered. The author concludes that the most appropriate devices for retrieving cases in any particular survey will depend largely upon the characteristics of the cohort and the resources that are available to each investigator. A common procedure should be to start with the least costly devices like postal and telephone services, and then to proceed to more costly services, like those of the National Credit Company; there are numerous alternatives between these two extremes. The author recommends that future authors of longitudinal surveys keep more complete records and routinely report their experiences on the devices they use.