The principles of fluorescent microscopy are discussed, together with the apparatus necessary for its study. Specially necessary is a lamp giving radiation in the ultraviolet, with filters to remove most of the visible light. Some histological structures have a natural fluorescence and may be studied directly. In other instances fluorescence is induced by the addition of various activating substances (usually dyes) known as fluorochromes. A list of commercial preparations of this sort is given, together with the type of fluorescence which they induce in various histological structures.