Substance misuse in the elderly population has been referred to as a silent epidemic. One of the factors that contributes to the appropriateness of this description is the difficulty of diagnosis in this age group. There is abundant evidence that substance misuse is under-diagnosed in elderly people and that this applies to both alcohol and drug misuse, although the general principles of a diagnostic approach are not age-related. The increased incidence of anxiety, depression, dementing illness, and physical illness in this population, independent of substance misuse, means that diagnoses can often be missed. The impact of co-morbidity (Figure 1) in older age groups is such that it remains the most important confounding factor in diagnosis. In this second section of the review we explore the range of psychiatric and physical illnesses that can coexist with, or be caused by, substance misuse in the elderly population. The issue of assessment as part of a treatment framework is considered in the third section of the review. The risk of missed diagnosis in this age group was illustrated in a study of diagnosis of substance misuse problems in patients aged 65 and over who had been admitted to hospital. Only three out of a total of 88 patients using benzodiazepines, 29 out of 76 smokers, and 33 out of 99 problem drinkers were correctly identified. Of those who were identified only a small proportion were referred on for specialist treatment.