Strong emission lines have betrayed the presence of strongly ionising radiation fields in some of the most distant galaxies in the Universe. These ionising radiation sources likely dominated the flux that led to cosmic reionization, and also drove the winds that enriched the intergalactic medium with metals, and may well have profound effects on their wider surroundings. While similar galaxies are rare in the local universe, they can be identified, in galaxies selected as analogues for the Lyman break galaxy population. Here I will discuss recent evidence for hard ionising potentials as a function of cosmic time, and also the state-of-the art models, such as BPASS which includes analysis of binary stellar evolution pathways, required for their interpretation.