The main needs of energy system models over the past three decades trace a journey from an era in which concerns about security and sufficiency of supply were the dominant themes (1970s and 1980s), through a swing towards liberalizing markets, particularly North American natural gas and electric power (1980s and 1990s), to a growing concern about climate change and greenhouse gas emissions (2000s). In addition, there are now numerous countries with quickly developing economies under central economic controls. Perhaps the future will require models that optimize the energy economies of such countries, developing under a centralized state capitalism model and administered prices. As policy imperatives evolved over time, so did the various models and their types, changing their techniques and evolving their data sources. As a result, there is now a plethora of different models that cater to the evolving needs of policy makers, including optimization, equilibrium, and macroeconometric models.