A classical case-control study design is frequently used in genetic epidemiology to investigate the association between genotype and the presence or absence of disease. Association studies can also be useful in the investigation of quantitative traits. The aim of such studies is to test for association at the population level between the quantitative trait and genotype at a particular locus. Whether investigating qualitative or quantitative traits, such studies depend on the prior identification of a candidate gene or genes. The genotyped locus could either be a polymorphism within a potentially trait-affecting gene or a marker in linkage disequilibrium with such a gene. Currently, screening of the whole genome is only feasible using linkage analysis, which is discussed elsewhere, because linkage extends over much greater distances than does linkage disequilibrium.