This special issue consolidates some recent research findings and scientific thought on co-occurring disorders from both the substance abuse and mental health fields. This summary article recaps and synthesizes the main findings and themes, then considers additional issues in the field today to arrive at an agenda for future co-occurring disorders research. Plans must: (1) encourage and assist further development of treatment programs that respond to an array of types and severities of co-occurring disorders while taking into account the limited resources typically available; (2) continue the development and testing of continuing care models by exploring strategies that will sustain the recovery of treated individuals who remain vulnerable to relapse; and (3) contribute to our understanding of the mechanisms and processes that enable new interventions and practices to be adopted, implemented, and sustained. “Co-occurring disorders” is a relatively new area of research; this special issue illustrates the productivity of work to date and indicates the potential for advances to come.