Over the past two decades a variety of national and international efforts has sought to bring together health and social scientists to address complex health issues. This paper reviews how the notion of transdisciplinary research has emerged; discusses research programs that have successfully traversed discipline boundaries in sustained fashion; considers facilitating and constraining factors that have emerged from the analyses of this process; and suggests next steps for conceptualizing, organizing, and assessing transdisciplinary research based on the notion of heterarchy.