Prostate cancer is the second most common malignancy among males after lung cancer. The growth of prostate cancer cells depends on the presence of androgens, a group of steroid hormones that include testosterone and its more active metabolite dihydrotestoste-rone. Most prostate cancers are androgen-dependent and respond to the antiandrogens or androgen-deprivation therapy. However, the progression to an androgen-independent stage occurs frequently. Possible mechanisms that could be involved in the development of hormone resistant prostate cancer causes including androgen receptor (AR) mutations, AR amplification/over expression, interaction between AR and other growth factors, and enhanced signaling in a ligand-independent manner are discussed.