The present dissertation project examined emerging adulthood in the context of substance use behavior. It is presently recognized that individuals pass through a transitional period between adolescence and young adulthood - "emerging adulthood." Continuation high school youth have demonstrated to show early entry into emerging adulthood-type developmental tasks. The first study examined the psychometric properties of the Inventory of Dimensions of Emerging Adulthood (IDEA) in a population of continuation high school students of Southern California. The IDEA was developed in an attempt to capture the psychosocial attributes of this unique developmental period. A 21-item version of the IDEA was evaluated, and it was determined that the measure was composed of three dimensions and demonstrated high internal consistency. In addition, construct validity was assessed and indicated that the measure, as expected, was correlated with demographic characteristics, risk behaviors, and other psychological measures. It was concluded that the IDEA is a practical instrument for measuring emerging adulthood in at-risk populations. Not every individual experiences emerging in the same fashion; emerging adulthood is characterized by its great heterogeneity. As such, the second study used a shortened 8-item IDEA measure to examine trajectories of emerging adulthood. The items were examined for factorial invariance across three time points of data collection. A shortened 5-item measure was found to be factorially invariant and was used in subsequent analysis. A latent growth curve model analysis indicated a sample that was growing in emerging adulthood status over time. Next, a latent class growth analysis was used to identify distinct trajectories. Three such trajectories were extracted from the data. These latent classes were then examined in terms of baseline characteristics as well as Time 3 behavioral outcomes. Large differences were found at baseline, but appeared to dissipate over the two-year period. These findings are expected to provide valuable implications for drug prevention programs directed at at-risk populations. It appears that this population is entering emerging adulthood earlier than other normative youth. As such, it might be beneficial to target these students earlier for risk behavior prevention efforts. Future research should examine emerging adulthood over a longer period of time, over additional developmental periods, and among diverse populations. If over longer periods of time differential outcomes are exhibited, it might be possible to distinguish these groups based on baseline characteristics, and perhaps to target those with the initial most risky trajectory.