Prostate cancer (CaP) is one of the most common malignancies in men, and the incidence of CaP is increasing. Because of the limitations of current therapeutic approaches, many patients die of secondary disease (metastases). Mucins are used as diagnostic markers as well as therapeutic targets due to their aberrant and unique expression pattern during cancer progression. There is a growing interest in mucins as treatment targets in human malignancies, including CaP. So far, 21 mucin genes have been identified. Of these, MUC1 has been investigated most extensively. In neoplastic tissues, MUC1 is underglycosylated compared with that in normal tissues. The reduced glycosylation permits the immune system to access the peptide core of the tumor-associated underglycosylated MUC1 antigen (uMUC1) and reveal epitopes that are masked in the normal cell. This feature makes it possible to design an antibody that discriminates between normal and adenocarcinoma cells and target tumor-associated MUC1 with toxins or radionuclides, or use a vaccine targeting tumor-associated MUC1 antigen. The results from our recent study have shown that over-expression of MUC1 plays a very important role in CaP progression and MUC1 is an ideal target for targeted therapy to control micrometastases and hormone refractory disease. This review will cover our current understanding of the structure and functions of MUC1, summarize its expression on human CaP tissues and focus on the MUC1-based immunotherapy for control of metastatic CaP.