Dual diagnosis is associated with frequent relapse, poor treatment engagement and overall unsatisfactory treatment outcomes. A comprehensive review of the contemporary literature examining this issue was conducted, finding a paucity of literature concerning dual diagnosis in older adults. Of the literature appraised for this review, a number of studies examined US Veteran's Affairs populations, which were largely male. Studies concerning older mental health populations were scarce. During the literature search, a number of background studies that influenced contemporary research regarding dual diagnosis in older adults were found; these studies were examined regarding their contribution to contemporary paradigms concerning older adults with co-occurring mental illness and substance use disorders. This review presents the results of the contemporary literature concerning dual diagnosis in older adults. Several recurring themes emerge from the literature, including the notion of a statistically small population that, in absolute terms, represents a sizeable number of individuals coming to the attention of aged mental health services in the future. Additionally, the potential for under-diagnosis in this cohort is highlighted, potentially creating a hidden population of older adults with dual diagnosis.