The use of higher technology in medicine promises improved outcomes and enhanced productivity. That is, successful techniques will lead to lower cost, higher quality care for a larger population. In surgery, these changes range from the more efficient use of skilled medical practitioners, through improvements to conventional practice (a recent example is the shift to endoscopic surgery in the abdomen), to the creation of new procedures which capitalize on the availability of information in new forms._Image Guided Surgery may be defined as the use of advanced technology to help the surgeon see with 1) Better resolution 2) Orientation and context setting 3) Higher contrast, and 4) Vision inside "solid objects", including the elimination of occlusion by the surgeon's tools or other external items. We describe here the current imaging processes and their limitations with regard to direct guidance of therapy. The physical properties of real time image acquisition systems are described along with the mechanisms for inherent and enhanced contrast. Examples of the use with surgical instruments or other interventional devices for image-monitored and guided procedures are then discussed, and future prospects elucidated.