The popularity of the confocal microscope in life science laboratories around the world is undoubtedly due to its ability to permit volume objects to be imaged and to be rendered in three dimensions. It is important to realize that the confocal microscope itself does not produce three-dimensional images. Indeed, it does the opposite. The critical property that the confocal microscope possesses, which the conventional microscope does not, is its ability to image efficiently (and in-focus) only those regions of a volume specimen that lie within a thin section in the focal region of the microscope. In other words, it is able to reject (i.e., vastly attenuate) light originating from out-of-focus regions of the specimen. To image a three-dimensional volume of a thick specimen, it is necessary to take a whole series of such thin optical sections as the specimen is moved axially through the focal region. Once this through-focus series of optically sectioned images has been recorded, it is a matter of computer processing to decide how the three-dimensional information is to be presented. There are many methods for producing optical sections, of which the confocal optical system is just one. This article reviews these methods and describes a number of convenient methods of implementation that can lead to, among other things, real-time image formation.