One of a series of sixteen knowledge transformation papers, this paper examines the advantages and disadvantages of longitudinal studies as a method for evaluating vocational programs. First, longitudinal methods are defined for the differences between them and cross-sectional studies are established. The results of a literature search conducted on the ERIC database are provided, showing that eighteen studies relevant to vocational education used longitudinal methods. Then, the importance of longitudinal data is emphasized because of its role in studying the relationships between vocational program practices and student development. In the next section, ways to implement longitudinal studies are discussed. The data requirements are found to include input information on biography and enrollment; process data, such as student progress and program characteristics; and output information on termination, initial placement, and follow-up. The conclusion acknowledges the problems associated with such studies to be the complicated process of data collection, the length of time involved, and the lack of prototypes for vocational educators to follow. References are cited at the end. (ELG)