The aim of this doctoral dissertation was to examine a conceptual model linking social withdrawal, maladaptive cognitions, and internalizing difficulties in childhood and emerging adulthood. In Studies 1 and 2, undergraduate students (Study 1: N = 451, Mage = 19.17, SD = 1.40; Study 2: N = 540, Mage = 19.20, SD = 1.49) completed assessments of social withdrawal (shyness, unsociability, social avoidance), maladaptive cognitions (threatening and negative cognitions), and internalizing problems (social anxiety, depressive symptoms). For both samples, results from structural equation modeling suggested partial specificity in the cognitive content associated with internalizing problems. Specifically, whereas threatening cognitions were positively associated with both social anxiety and depressive symptoms, negative cognitions were only positively associated with depressive symptoms. Differences in the social, cognitive, and emotional implications of social withdrawal subtypes also emerged across studies. In Study 1, results suggested that maladaptive cognitions played a mediating role in the links between shyness (but not social avoidance) and internalizing problems. In a conceptual replication of Study 1, results from Study 2 indicated that threatening cognitions mediated the effects of both shyness and social avoidance on social anxiety and depression; however, shyness and social avoidance only displayed indirect effects on depressive symptoms via negative cognitions. The goal of Study 3 was to explore whether a similar model linking withdrawal, peer relations, social cognitions, and internalizing problems could be applied to a sample of early elementary school-aged children (N = 408 children, Mage = 7.10 years, SD = .86). During individual interviews, children completed assessments of their social cognitions (rejection sensitivity, negative social information processing), and parents provided assessments of children's socially withdrawn behaviours, socio-emotional functioning, and peer relations. Among the results, peer problems (but not social cognitions) emerged as a significant mediator in the associations between both shyness and social avoidance and internalizing problems. Results are discussed in the terms of the meaning, as well as the social, emotional, and cognitive implications of different subtypes of social withdrawal at different developmental periods. Future directions and potential implications for intervention and prevention development are also discussed.