We discuss applying single-photon detectors to measurements with high accuracy. Two primary standard methods of detector calibration are reviewed: one based on a radiant power measurement (substitution method) and the other—on a simultaneous generation of photon pairs (correlation method). It is shown how to apply these methods to two types of detectors: with no photon number resolution (PNR) and with full PNR. In addition, an experimental comparison of the two calibration methods with non-PNR detectors is presented.