Abstract--Genetic causes of disease, including stroke, range from classic mendelian (a single gene leads to disease) to complex (multiple genes contribute to risk for disease in combination with other genetic and/or environmental factors). One method of identifying genetic risk factors is the candidate gene association study, in which a given polymorphism in a gene of interest is compared between cases or controls; if the polymorphism is more common in affected subjects, a contribution to risk for disease is implied. A candidate gene is usually selected because the gene product is intuitively related to the disease process. Further studies in other populations will reveal whether findings from both association and candidate gene studies can be generalized. Because stroke includes multiple clinically relevant subgroups, the appropriate phenotyping approach is not yet known. It also is unclear whether the biological factors contributing to risk for one type of stroke are the same as those for the other types or whether these are biologically differing entities. Medical risk factors for stroke, such as hypertension, and shared risk factors between stroke and cardiovascular disease, for example, suggest that biological processes overlap across related disorders. Despite the complex biological questions that remain to be answered regarding stroke etiology and risk, genetic studies are of value to elucidate risk factors for stroke, give us clues regarding targets for interventional therapies, and give us insight into the process leading to clinical stroke of all types.