The main objective of this thesis is first to adapt energy models for the use in developing countries and second to model sustainable energy transitions and their effects in rapidly developing countries like China and India. The focus of this thesis is three-fold: to elaborate the differences between energy systems in (rapidly) developing countries and in industrialised countries; to adapt energy modelling approaches to improve their suitability for rapidly developing countries; and to develop scenarios using these adapted models to simulate sustainable energy transitions for rapidly developing countries and to assess the implications of these energy transitions. The core of this research is to develop scenarios for sustainable energy transitions for China and India. China and India are chosen, because they are currently considered the most rapidly developing countries, and thereby also the most energy-consuming, most climate-relevant and most economically-growing developing countries. Their development is expected to have global impacts. Implementing renewable and clean energy sources in these countries is likely to mitigate climate change.