The success of dose escalation strategies in prostate cancer over the last decade has relied on high accuracy in target delineation, localization and radiation delivery. Improved biochemical control with acceptable levels of toxicity (specifically rectal) has been achieved by stringently monitoring the location of the prostate during the course of radiation treatment. Studies in the early 1990's demonstrated that daily bony alignment was a poor surrogate for prostate gland location. Since then, various strategies have been devised to localize the prostate precisely during treatment. Of these, Transrectal Ultrasound (TRUS)-guided gold fiducial marker (FM) insertion and x-ray imaging of markers has been a broadly successful approach. TRUS guidance has been used for fiducial marker insertion within the prostate since 1985 in various centers throughout the world and has also been a standard practice in PMH since 1997 in men undergoing radical external beam radiotherapy. A retrospective comparative study of 106 patients evaluating the relative accuracy of endorectal MRI and TRUS in detecting the location of tumor reported an improved performance of endorectal MRI especially in the base and midgland regions. Over the last few years, mounting experience in the interpretation of prostate MRI, and addition of physiologic imaging sequences has further improved the performance of MRI in detecting and localizing the GTV.