Nebular emission lines are frequently used to understand the history of star-formation and conditions of gas and dust within galaxies. However until very recently modelling of emission lines has assumed all stars are single. We know today that most stars are in binary systems and interactions in these can lead to stars being stripped of their hydrogen-rich envelopes to expose the inner core, forming helium-rich and helium stars. Due to their mean-molecular weight these stars are more compact and hotter than most stars and emit significant amounts of ionizing radiation. I will discuss how taking account of these stars changes how we understand emission lines within galaxies. I’ll finally cover other binary sources we need to consider and investigate including X-ray binaries.