Understanding the biology of prostate cancer and the roles of androgen receptor in prostate cancer progression is essential to the development of novel therapeutic strategies to effectively attack and eradicate this disease. Preclinical, in vivo, studies are critical to further evaluate potential clinical relevance of in vitro findings. Ideally, in vivo studies should employ models that mimic characteristics of prostate cancer from early diagnosis through the period of castration-resistant metastases. In this chapter we describe methodologies used to grow human prostate cancer xenografts in mice. In this setting, roles of androgen receptor signaling in prostate cancer progression and efficacy of novel treatment modalities, including those affecting androgen receptor signaling, can be investigated.