Fuel cells have become a very important part of the energy supply in past, current and future space activities. In the past they have been used in the Gemini missions - their first application in space - and later in the Apollo programme. Fuel cells will be employed in future American, Russian and European space vehicles where significantly larger electrical power generation is envisaged compared to early applications.This increased capacity is linked to a correspondingly larger amount of waste heat which must be managed in turn. Effective study of how best to accommodate such fuel cell systems into a space vehicle and of the impact on other subsystems - e.g. thermal control - requires a suitable simulation tool.