<p>Over the past several decades, evidence has accumulated to support a transdiagnostic model of some of the processes underlying mental illnesses—that there are dimensional variations in genetic, environmental, neurobiological, and psychological factors that contribute to shared aspects of risk for developing psychiatric disorders, the majority of which emerge during childhood, adolescence and early adulthood. In this narrative review, the multiple, convergent lines of evidence for this understanding of psychiatric illness are summarized, and an integrated model of this evidence for both shared and non-shared risk factors and manifestations of psychiatric illnesses is proposed. This model can provide one testable framework for future investigations and a rationale for the development and dissemination of transdiagnostic approaches to mental illness prevention.</p>