The treatment of prostate cancer, confined to the gland, with high conformal doses to the target volume and sparing of bladder, rectum and urethra at the same time can be achieved by interstitial brachytherapy. Close cooperation of urologist and radiologist, together with the physical treatment planning have improved the clinical results significantly. The ongoing development of new radioactive isotopes and of dedicated computerized treatment planning systems have resulted in a renaissance of the interstitial treatment. "Preplanning" i.e. preoperative treatment planning can be performed precisely and fast. Improved ultrasound techniques allow during the perineal template guided seed implantation a realtime dose calculation resulting in an improved seed placement. CT- or MRI based "postplanning" guarantees for early postoperative dose documentation and quality assurance. 2-dimensional as well as 3-dimensional dose distributions superposed to anatomical structures and dose volume histograms (DVH) allow for dose optimization and quality decision. 192Ir with high activity is used for the high dose rate (HDR) afterloading treatment as a boost after external radiotherapy. 125I and 103Pd as well are used as permanent implants to boost external irradiation. If fast growing low grade tumors should be treated with 103Pd permanent implant and slow growing tumors with 125I is discussed controversialy. About 50 to 100 seeds are implanted. The implanted patients are allowed to leave the hospital as there is sufficient shielding by the surrounding normal tissue. Postplanning is based on CT- or MRI for dose documentation and quality assurance. For 125I treatments the activity-dose-relation was redefined since 1995 (TG43 protocol). Similar corrections seem to be necessary for 103Pd treatments.