Prostate specific antigen (PSA) is a serine protease present in an enzymatically inactive form in human serum at a concentration of nanograms/ml; serum PSA (S-PSA) is used widely as a surrogate marker in the diagnosis and management of prostate cancer (CaP). However, the vast majority of PSA produced in the prostate is secreted as active enzyme into the seminal fluid or is sequestered within the prostate tissue microenvironment. This book examines the role of the prostate-specific antigen in prostate tumour growth.