SKA1 - Low is proposed to be a low-frequency aperture array, operating at ~ 50MHz - 350MHz. The primary science case for SKA1-low is detection of the 21-cm HI-line during the Epoch of Reionization at redshifts from between 5 and 25. Secondary science cases include HI-line absorption against continuum sources, low radio frequency observations of pulsars, and magnetized plasmas both in the Galaxy and intergalactic space. This talk will briefly summarise these science cases and associated requirements, focussing on the Epoch of Reionization.