Many genetic disorders do not manifest themselves until the adult years. Such disorders often involve multiple genetic factors interacting with multiple environmental factors, over time, to produce a phenotype. This paper reviews the modes of inheritance of genetic disorders and describes the types of genetic testing that are currently available. It offers clues that should lead physicians to suspect that an adult patient might have a genetic disorder and raises issues that should be considered in counselling the patient about genetic testing. Resources for patients and their family physicians are also discussed.