The development of gas turbines using 100% hydrogen as fuel is an important step towards the development of new energy and propulsion technologies using zero carbon fuels. This chapter reviews some elements of combustion science and engineering that can help with these developments. Stable and low-NOx hydrogen combustors face significant challenges. Stabilisation of the flame at the right location without autoignition or flashback, with low NOx production, and without any thermoacoustic oscillations is important to achieve. Some new combustor architectures addressing these requirements are reviewed. The importance of mixing history is emphasised and some novel tools that can be used for assessing the effects of mixing on NOx are discussed.