This paper describes the nature and consequences of co-morbidity, as applied to co-occurring mental health and alcohol/other drug (AOD) use problems. The 'co-morbidity roundabout' is introduced as a useful metaphor for conceptualizing the current experiences of people with co-occurring mental health and AOD use problems. In order to successfully negotiate the 'roundabout', the 'drivers' (people with co-morbid mental health and AOD use problems) must consider a range of internal and external conditions (knowledge about services, support from family, friends, health providers, motivation to change, etc.), account for their vehicle's characteristics (other conditions and demands, including social/legal/financial issues), keep their travel itinerary in mind (plans for change including treatment) and navigate through the many detours and dead-ends that they may confront (eligibility for services, accessibility of treatments, etc.). Co-morbidity is a major contributing factor in 'drivers' failing to successfully negotiate, or even becoming 'stuck' on, the 'roundabout'. A summary of relevant treatment research is also presented, including descriptions of brief interventions and more intensive treatment approaches. Finally, the 'co-morbidity roundabout' metaphor is expanded to assist clinicians to translate the findings from this treatment research into clinical practice. Further suggestions are made for improved navigation through and exit from the 'roundabout', including recommendations for the use of a stepped-care approach to the assessment and treatment of clients with co-morbid mental health and AOD use problems.