Longitudinal studies are commonly used in the social and behavioral sciences to answer a wide variety of research questions. Longitudinal researchers often collect data anonymously from participants when studying sensitive topics to ensure that accurate information is provided. One difficulty gathering longitudinal anonymous data is that of correctly matching participants across waves of data collection. A number of methods have been proposed for using nonidentifying codes to match anonymous participants; however, currently there is no consensus on the most effective method. This article reviews and analyzes the literature on nonidentifying codes and provides recommendations for researchers interested in using these types of codes in conducting anonymous longitudinal studies.