The objective of this dissertation is to explore the nature of behavior problems at crucial stages of development. Following a brief overview of autism spectrum disorder (ASD) and intellectual disability (ID), including the expression of co-occurring behaviors in these populations, two related studies are presented. Both studies utilized the instruments of the Achenbach System of Empirically Based Assessments (ASEBA; Achenbach & Rescorla, 2000; 2001) to describe behavior problems, as observed during early childhood to adolescence. Study 1 utilized a sample of young children, ages 5 years and under, deemed at risk of a developmental disorder who were referred to a university-based screening clinic to be assessed for possible ASD. The objective of Study 1 was to empirically identify homogenous classes of emotional and behavioral syndromes during early childhood. Using latent class analysis, results revealed that behaviors clustered into two classes: a highly behavioral class characterized by emotion dysregulation difficulties (56%) and a normative class (44%). Predictors of autism classification and severity were included in the analysis but yielded no significant effects. These findings highlight the need to assess emotional regulation issues prior to formal schooling and to provide emotion-focused interventions that may foster the use of effective coping strategies. Study 2 utilizes a sample of children and adolescents with and without ID from a NICHD-funded longitudinal study (PIs: Bruce L. Baker, Jan Blacher, & Keith Crnic). The aim of Study 2 was (a) to examine the trajectory of behavior problems from ages g to 15 and (b) to explore whether a transactional relationship exists between behavior problems and social skills over these time points. Controlling for disability status, results of a fitted path analysis model revealed that ratings of behavior problems (internalizing and externalizing) and social skills were highly stable over time. A behavior-driven model fit the data best and revealed that behaviors in early adolescence significantly contributed to later social difficulties. Findings illustrate how challenging behaviors may impact emerging social competence during adolescent years. Implications for practice and future research directions are discussed.