A suitably equipped microscope provides a ready means of making measurements which in some cases could otherwise be achieved only with considerable difficulty. Such measurements are of two main types: those where the object is small enough to be contained within the microscope field, and those of such a size that either object or microscope must be moved to bring the extremities of the object successively to the center of the field of view, the center being marked by a pair of cross lines or a target. This article reviews the instruments and measurement applications.