Recent models of adolescent development and psychopathology emphasize the importance of the social regulation of emotion during adolescence (Allen & Manning, 2007; Allen & Miga, 2010), and emotion regulation as a mediating factor between multiple aspects of adolescent development and adolescent adjustment (Morris, Silk, Steinberg, Myers, & Robinson, 2007). The present dissertation investigated these two phenomena in two separate studies of adolescent development, emotion regulation, and psychological adjustment. In study one, a new measure of adolescent social regulation of emotion – the Managing Distress Interpersonally Scale, or MANDI – showed good internal consistency, test-retest reliability and factor structure across two samples (217 college students in sample one and 63 community adolescents in sample two). The MANDI also showed good convergent and discriminant validity in its relations with independent assessments of adolescent emotion regulation, relationship functioning, psychological functioning, and physiological regulation of emotion. In study two, 64 community adolescents completed self-report, interview, and physiological procedures (salivary cortisol and respiratory sinus arrhythmia), while one of their parents also completed survey measures. Emotion regulation was found to mediate the developmental context and adolescent depressive symptoms, alcohol problems, and peer aggression. Findings are discussed in terms of the utility of intervening at the level of emotion regulation for adolescents displaying difficulty with internalizing and externalizing symptoms.