We review both the theory and the data supporting the neo-Darwinian claim that adaptation results from the accumulation of many alleles of small effect. The theory supporting this "micromutationist" view is inconclusive. Furthermore, the evidence from genetic analysis is mixed: although some adaptations are apparently based on many genes of small effect, others clearly involve major genes. Most genetic analyses, moreover, have lacked the power to detect major genes. We conclude that, although the neo-Darwinian view of adaptation may be correct, it is not strongly supported by evidence. The introduction of molecular approaches may, however, finally allow us to resolve this question.