Mathematical models of energy-economy-environmental systems (E3) provide a rational framework for exploring the effects of energy and climate policies and support adequate decision-making. Numerous models have been developed over the years with different solution approaches, features, geographical scope and time resolution. There is no complete or ideal models but different models that answer different questions or similar questions with different perspectives. Developed since the early 1980s, the TIMES (The Integrated MARKAL-EFOM System) optimization models have contributed to support decision-making at various geographical scales from global to city levels. In this Chapter, we distinguish a set of national studies that performed TIMES model developments to study the energy transition and address the impacts of integrating high levels of renewable energies on the system. Each study follows a different approach with the sole purpose to optimize the energy used in order to reduce greenhouse gas (GHG) emissions. Examples of applications are provided to illustrate the rich potential of optimization models for assisting decision makers with climate change mitigation. In particular, a special attention is given to the electricity sector as electrification of end-uses and decarbonization of the electricity sector are consistent priorities of actions across studies.