An imaging spectrometer is a device for examining the 3 dimensional parameter space (x, y, X) which characterizes a major part of the information of interest about astrophysical sour-ces. There are many ways in which this 3D space may be examined, and the techniques used vary across the electromagnetic spectrum according to the wavelength, and hence the energy of a photon which dictates the telescope diffraction limit and the relevant detector and spectrometer technology. In the visible region of the spectrum the advent of high quantum efficiency, linear, panoramic detectors has led to the development of spectrometers coupled to these detectors, and consequently a large gain in the speed with which the parameter space may be explored. There are three basic types of spectrometer in general use in the visible, based on diffraction gratings, and Fabry-Perot and Michelson interferometers. The last two possess the well known Jacquinot advantage, allowing a much greater throughput of light at a given resolution than a grating of similar size. However, when we are interested in the spectral as well as the spatial information this advantage needs to be reassessed. The use of 2D detectors allows us to access 2 of the three dimensions simultaneously. In the first part of this paper we compare these three types of imaging spectrometer (see Fig. 1) in terms of their ability to obtain both spectral and spatial information, and discuss practical limitations. The second part describes the the new Imaging Fabry Perot being designed for the 4.2 m on La Palma. The final section deals with the conditions under which optimum use is made of the different devices.