The intensity of individual pulses varies randomly over various time scales. On the shortest time scale, pulse to pulse intensity fluctuations are thought to be due to intrinsic processes in the pulsar magnetosphere. Longer time scale fluctuations in the mean pulsar flux are produced by propagation processes in the ionised plasma of the interstellar medium (ISM). Furthermore, some of these intensity fluctuations can be uncorrelated over large frequency intervals. Thus for purposes of estimating the pulsar flux (including estimates of the spectral index) and for studying the variations in the pulsar flux to understand properties of the ISM, pulsar observations need to be calibrated with known sources of power. This can be done by using either calibrated noise sources that can be switched into the signal path or known calibration sources in the sky.