A device for guiding surgical tools by ultrasonic imaging, which device comprises: a) means for acquiring in real time a time sequence of 3D ultrasonic images of a target area; b) means for determining and/or tracking, at the same time as step a) and in real time, the position and orientation of a surgical tool and means for defining the position or direction of a characteristic axis of the tool corresponding to the detected position and orientation of the tool; c) means for determining in real time the position of a working or functional end of the tool along said direction of said characteristic axis; d) means for determining in real time the relative position in space of each real time 3D image and the direction of the characteristic axis of the tool corresponding to each real time 3D image; e) means for generating, for one or more of the real time 3D images acquired in a time sequence, a real time 2D image defined by an image plane that intersects the corresponding real time 3D image of said time sequence, which real time 2D image plane has a predetermined inclination relative to the direction of the characteristic axis of the tool determined in real time and a predetermined distance from the working end of the tool with reference to the orientation and position of the tool upon acquisition of the corresponding real time 3D image, said real time 2D image being generated using the image data of the corresponding real time 3D image of the time sequence of real time 3D images; g) the inclination of said real time 2D image plane relative to the direction of the characteristic axis of the tool determined in real time, and the distance of said 2D image plane relative to the working end of the tool being such that the image generated in the 2D image plane corresponds at least approximately to a vision of the target area from the point of view of an observer situated at or near a part of the tool or said characteristic axis of the tool; h) the image generated along said plane including indications of the position of the target to be treated by said tool and/or the position of the characteristic axis of said tool in said image and the dimensions of the tool in a direction perpendicular to said characteristic axis. a) means for acquiring in real time a time sequence of 3D ultrasonic images of a target area; b) means for determining and/or tracking, at the same time as step a) and in real time, the position and orientation of a surgical tool and means for defining the position or direction of a characteristic axis of the tool corresponding to the detected position and orientation of the tool; c) means for determining in real time the position of a working or functional end of the tool along said direction of said characteristic axis; d) means for determining in real time the relative position in space of each real time 3D image and the direction of the characteristic axis of the tool corresponding to each real time 3D image; e) means for generating, for one or more of the real time 3D images acquired in a time sequence, a real time 2D image defined by an image plane that intersects the corresponding real time 3D image of said time sequence, which real time 2D image plane has a predetermined inclination relative to the direction of the characteristic axis of the tool determined in real time and a predetermined distance from the working end of the tool with reference to the orientation and position of the tool upon acquisition of the corresponding real time 3D image, said real time 2D image being generated using the image data of the corresponding real time 3D image of the time sequence of real time 3D images; g) the inclination of said real time 2D image plane relative to the direction of the characteristic axis of the tool determined in real time, and the distance of said 2D image plane relative to the working end of the tool being such that the image generated in the 2D image plane corresponds at least approximately to a vision of the target area from the point of view of an observer situated at or near a part of the tool or said characteristic axis of the tool; h) the image generated along said plane including indications of the position of the target to be treated by said tool and/or the position of the characteristic axis of said tool in said image and the dimensions of the tool in a direction perpendicular to said characteristic axis.