Different types of nonresponse threaten the validity of longitudinal studies: first,initial nonresponse during the recruitment in the baseline survey and second, successivedropout at each time point. Dropout in longitudinal surveys has three separate sources:failure to locate research participants, failure to contact participants, and failure to achievecooperation. In this entry, specific attention is given to existing knowledge on the corre-lates of dropout and strategies to limit the problem. To limit nonresponse, a total designapproach is advocated with specific attention to each source. That is to limit both noncon-tact (i.e., failure to locate and subsequent failure to contact a located research participant)and noncooperation.