There are many long-term energy models currently in use with different underlying principles, characteristics, inputs, and outputs. Over the past 30 years, considerable efforts have been made to develop new models, following different approaches that vary in terms of model starting point and on the type of questions they are designed to answer. These models focus on the period to 2050 and to 2100 and are used to build future energy scenarios to assess the impacts of policy decisions and to build a rich knowledge base for climate change and energy security policies. PET, TIAM, and MESSAGE (examples of energy system models) provide a range of technology detailed energy system configurations to deliver future energy service demands at least cost. By contrast, GEM-E3, GEMINI-E3 and, GTEM (examples of general equilibrium models) are also optimization models, but here the energy system is less detailed but the whole economy is modeled. Other models (e.g., POLES) simulate the future evolution of energy demand and supply and a number of models have been developed that include energy, but which focus on integrated assessment of the economy energy and climate response (e.g., FUND, GCAM, and MERGE). It is important for energy analysts to assess what type of model best suits the requirement and to recognize the limitations of the various models available. Significant work is required to improve linkages between models to harness respective strengths and major modeling gaps (agriculture, land use, and behavior) require a specific focus for future work.