The ApoTome optical sectioning device from Carl Zeiss was designed to make it fast and easy to generate optical sections of biological fluorescence specimens. This optoelectronic add-on device for a fluorescence light microscope uses structured illumination to reject the signals belonging to regions of the sample that are outside the best-focus position of the microscope so that the fluorescence that is generated in those regions is not displayed in the image. The pattern of a transmittance grating is projected into the specimen, superimposing dark fringes that are visible through the eyepiece and through a CCD camera. A slider is inserted into the field diaphragm slit of the microscope to carry the grating and to move the projected fringes into one direction (X) of the sample plane (X-Y). A sequence of images is acquired and stored in the PC memory, and the position of fringes is altered between each image to determine the amplitude of intensity modulation of any point in the observed sample plane. The system produces one-Airy-unit optical section depth, indicating high optical resolution and a good signal-to-noise ratio. Visible resolution in the Z-direction is increased by a factor of two as compared with conventional fluorescence microscopy, allowing researchers to display 3-D images of optical sections collected from the full sample, even with thick specimens. Applications for the system include neuro- and cell biology and developmental biomedical research.