The determination of genetic polymorphisms for susceptibility to human disease has been rapidly increasing since the introduction of the polymerase chain reaction (PCR). In most laboratories the ability exists to conduct studies on more than 10,000 persons, and the prospect of even larger investigations is approaching. Many methods can be used for genotyping individuals but some are more common and less expensive than others. Newer methods will allow for automation. As the number of studies on genetic polymorphisms increases it is to be expected that more pitfalls will be encountered. While larger studies will reduce the importance of misclassification, quality control methods will have to be applied to the processing of large numbers of samples.