Androgen receptor (AR), a key nuclear transcription factor in the prostate gland, is expressed in all histological types and stages of prostate cancer. The AR regulates proliferation of prostate cancer cells by stimulation of cyclin-dependent kinases. However, in some prostate tumors AR stimulates expression of cell cycle inhibitors, thus leading to down-regulation of cellular proliferation. Androgens, by activation of the AR, control differentiation of prostate cells and synthesis of neutral lipids. There are several mechanisms by which prostate cancer cells adapt to an environment with low androgen supply during endocrine therapy. The AR expression and activity increase in several cell lines that are used as an in vitro model for monitoring changes during long-term androgen ablation. Mutant ARs are of importance for monitoring the natural course of the disease and for determining the response to anti-androgens in metastatic lesions from prostatic carcinoma. In addition, AR activity is up-regulated by various stimulators of intracellular protein kinases. Current research efforts are focused on elucidation of function of AR coregulatory proteins, coactivators and corepressors. Their inappropriate expression and/or function might critically influence cellular events in advanced carcinoma of the prostate. It is hoped that information on these coregulatory proteins will serve as a basis for a more efficient pharmacological inhibition of the AR in advanced carcinoma of the prostate.