A dominant paradigm in psychopathology research proposes that individual differences in personality are centrally involved in the origins and manifestations of psychopathology, and structural models of personality and psychopathology have been extremely useful in helping to organize associations among many traits and disorders. However, these models merely describe patterns of covariation; they do not explain the processes by which these patterns emerge. We argue that the field is stagnated, as it is overly focused on the demonstration of concurrent associations and on confirming a spectrum model that proposes traits and disorders are manifestations of the same underlying constructs. We contend that if the field is to move toward an understanding of causal processes, it must integrate knowledge and principles of personality development and developmental psychopathology. To begin this integration, we review (1) normative trends in personality change, (2) age-related changes in the prevalence of disorders, and (3) the impact of onset and chronicity on the severity of disorders. We propose several developmental processes that may contribute to the co-development of personality and psychopathology. We then present novel empirical findings to illustrate how a developmental perspective on traits and disorders can inform new hypotheses, and propose principles and hypotheses that should guide future research.