The search for a more sensitive and specific marker for prostate cancer as well as for use in forensic investigations led to the discovery of PSA. Utilization of the purified protein resulted in the development of many immunoassays that are now in widespread use and have been applied successfully to the detection and monitoring of prostate cancer. Subsequent investigation into the biochemical characteristics of PSA revealed free and complexed forms of PSA in serum whose measurement now may provide additional clinical benefit. Further research into the molecular and biochemical properties of PSA as well as improvements in measurement techniques no doubt will lead to new applications and enhanced clinical use for this important tumor marker.