Prostate cancer is the most common cancer in men and the second leading cause of cancer-related death in men. There is a critical unmet need for predictive biomarkers to guide prostate cancer therapy in settings ranging from localized to metastatic disease. In metastatic prostate cancer, multiple FDA-approved therapeutic options are now available, but we lack non-invasive biomarkers that can predict treatment responses in each individual patient. In localized prostate cancer, reliable biomarkers are needed to help guide the rational selection of appropriate management options tailored to each patient. A major focus of our laboratory is the investigation of circulating tumors cells (CTCs) in the blood, which can be sampled non-invasively and analyzed to reveal molecular signatures that can predict and monitor treatment efficacy in patients receiving prostate cancer targeted therapies. Ongoing projects include the development of CTC-based and other blood-based biomarkers of radiation treatment response and treatment-related toxicity.