MSE is an 11.25m telescope with a 1.5 sq.deg. field of view. It can simultaneously obtain 3249 spectra at R = 3000 from 360-1800nm, and 1083 spectra at R = 40000 in the optical. The large field of view, large number of targets, as well as the use of more than 4000 optical fibres to transport the light from the focal plane to the spectrographs, means that precise and accurate science calibration is difficult but essential to obtaining the science goals. As a large aperture telescope focusing on the faint Universe, precision sky subtraction and spectrophotometry are especially important. Here, we discuss the science calibration requirements, and the adopted calibration strategy, including operational features and hardware, that will enable the successful scientific exploitation of the vast MSE dataset.