Interpreting the photometry and spectroscopy of galaxies often requires consideration of their integrated light. For this, models which accurately represent the emission from a stellar population as a function of its age, metallicity and other properties are essential. Such stellar population modelling is a mature field where old, metal-rich stellar populations are concerned. However galaxies exist at all redshifts which are much younger and lower in metal enrichment than a typical local galaxy. Amongst distant galaxies, or the hosts of stripped-envelope transients, such systems become the norm. Modelling these population requires a detailed understanding of massive star evolution and its effects on the integrated light. In particular, it has become clear that interactions between stellar binaries and other multiples may have significant effects on the observed population. Here I will discuss the current status of stellar population synthesis modelling and the remaining uncertainties, particularly with respect to multiple star evolution and factors affecting emission line galaxies in the distant Universe.