Abstract: The present invention relates to a method and apparatus for registering three-dimensional ultrasound images to an external coordinate system, such as the coordinate system attached to a camera-based imaging system or the coordinate system used to define the movements of a surgical robot. The apparatus generally comprises a tool with markers that can be accurately and repeatably located in both the ultrasound and external coordinate systems. In the case of the surgical robot frame, this tool may be a component of the surgical robot itself, such as a portion of a surgical manipulator.