The ability to obtain quantitative measurements from the visual analysis of specimens is a growing requirement in many branches of science and engineering. The following paper describes the opto-electronic system used in the Ealing-Beck Histotrak and Optomax image analysis instruments. The system incorporates a CCTV system linked to an optical input device for formatting the image of the specimen for analysis onto a TV camera and an electronic processing sys-tem for analysing the resulting video information. A light pen and associated 80K bit store is included in the system. The light pen, which is used to 'draw' on a video monitor displaying the TV camera image, may either define the boundary of an area of the displayed specimen within which analysis is required or be used to program the movement of the specimen stage. The system store may also be used to retain detected feature information over the whole field of view, allowing comparison of subsequent features with previously detected and stored information.