Abstract: A system for assisting guidance of a tool manipulated by a practitioner relative to a portion of the human body. The system includes elements for making a three-dimensional first image of a first scene including the human body portion and a first reference associated therewith, elements for making a three-dimensional second image of a second scene including the tool and a second reference associated therewith, the second image being made relative to the first reference, elements for making a third image constituted by superposing the first and second images so that the representations of the first reference contained respectively in the first and second images substantially coincide, and elements for projecting the third image into the field of view of the practitioner's eye while manipulating the tool.