Prostate cancer is the most common malignancy affecting men in the United States. Traditional therapy with radical prostatectomy or radiation therapy can be curative for localized disease, but metastatic prostate cancer is currently incurable. The only treatments known to prolong survival in patients with metastatic disease are androgen-deprivation therapy and chemotherapy, both of which have significant side effects. Immunotherapy approaches offer hope in providing new treatments to delay disease progression, ideally with fewer side effects. The results from nearly all early immunotherapy clinical trials for prostate cancer conducted to date have shown minimal toxicity, and many have suggested clinical benefit in terms of delaying disease progression. Several phase III clinical trials are currently under way in patients with metastatic, androgen-independent prostate cancer. The current article reviews recent trials evaluating immune-modulating agents, antigen-specific active immunotherapy, and combination therapies in clinical development for the treatment of prostate cancer.