This article presents a group-based method to jointly estimate developmental trajectories of 2 distinct but theoretically related measurement series. The method will aid the analysis of comorbidity and heterotypic continuity. Three key outputs of the model are (a) for both measurement series, the form of the trajectory of distinctive subpopulations; (b) the probability of membership in each such trajectory group; and (c) the joint probability of membership in trajectory groups across behaviors. This final output offers 2 novel features. First, the joint probabilities can characterize the linkage in the developmental course of distinct but related behaviors. Second, the joint probabilities can measure differences within the population in the magnitude of this linkage. Two examples are presented to illustrate the application of the method.