The principles for defining, comparing and calculating the signal-to-noise ratio performance of imaging spectrometers are presented. The relative signal-to-noise ratios (SNRs) of the main classes of imaging spectrometer are discussed both in general terms and with an emphasis on real-time, low spectral resolution applications. This general analysis is based on some simplifying assumptions and SNRs are also calculated for a typical application without these assumptions. These SNRs are compared to the signal-to-noise ratios typically required in imaging spectrometry. It is shown that for low resolution imaging spectrometry of low radiance scenes there are only small differences in SNR between the four main classes of instrument. For high spectral resolution imaging of low radiance scenes Fourier-transform techniques offer higher SNRs, but for high radiance scenes the impact of detector saturation tends to favor direct imaging spectrometry. It is noted however, that real-time, temporally scanned, imaging spectrometry requires track and stare stabilization to fully realize its potential.