This lecture presents a novel Augmented Reality (AR) probe and method for use with image-guided surgical navigation systems. Its purpose is to enhance existing systems by providing a video image of the therapeutic site augmented with relevant structures preoperatively defined by the user. The AR probe consists of a video camera and a tracked reference plate mounted on a lightweight ergonomic casing. It directly connects to the navigation tracking system, and can be hand-held or fixed. The method automatically updates the displayed augmented images to match the AR probe viewpoint without additional on-site calibration or registration. The advantages of the AR probe are that its simplicity of use close to the clinical pratice, adaptability to changing viewpoints, and its low cost. Our in-vitro experiments show that the accuracy of targeting under AR probe guidance is on average 1.9 mm (std=0.45mm), which is suitable for a wide variery of treatments.