According to Wohlwill (1973), developmental trajectories can be described in terms of a variety of parameters indicating presence of change; direction of change; shape of trajectory; values of maxima, minima, or terminal levels; sequencing of events; timing of events; and/or age corresponding to specified values of any of those characteristics. The sheer number of possible parameters serves as a reminder that whichever one is used in empirical studies can only provide a very incomplete picture of individual differences in developmental trajectories. The empirical study of such parameters can be aimed at two somewhat different objectives. First, researchers may be interested in identifying the factors and mechanisms that produce individual differences in a parameter of interest. Second, it is also desirable and useful to examine whether differences in a given parameter predict differences in relevant developmental outcomes at a later point in time. In this chapter, we are primarily concerned with the second objective. Ideally, the two approaches complement each other by focusing on the same or similar parameters. Unfortunately, however, theories of substance abuse and dependence provide little guidance as to which of the many available parameters are more or less useful for an understanding of the development of abuse and dependence in young adulthood.