Therapy of tumors needs detailed planning using CT or MRI to show extension of the malignoma, define intended safety margins and point out vital structures. Furthermore reconstruction following tumor resection needs reliable information also to choose correct type of grafts and to predict the outcome. On the base of an axial spiral CT or MRI data set the STN-Navigation-System (Stryker-Leibinger/Zeiss) is used for preoperative planning, intraoperative navigation and postoperative controlling of radical tumor resection and reconstruction. The predictive and intraoperative advantage concerning postsurgical outcome by using modern navigation systems is investigated in the treatment of patients with carcinoma, osteoma, meningioma and esthesio-neuroblastoma. Tumor resection and primary reconstruction were preoperatively planned. Intraoperatively tumor, savety margins and contours of transplanted tissue were navigated. Frameless stereotaxy was successfully completed in all cases of radical tumor resection. Computer assisted treatment improves preoperative planning by combining CT and MRI data. Tumor volume before preoperative chemotherapy can be assessed and transfered to the data-set after chemotherapy to perform radical resection inside the old borderlines. Intraoperative navigation makes radical tumor surgery more reliable by going for the safety margins, saving vital structures and leading the reconstruction to preplanned results. Keywords: computer assisted surgery, frameless stereotaxy, maxillofacial surgery, reconstructive surgery