With the emergence of new energy-related technologies and new energy sources, energy planning has become even more vital and complex. Decision making and optimization are very important for complex energy systems. Efficient decision making requires the involvement of various stakeholders which makes the decision problem even more difficult. Fuzzy sets provide tools for mathematically representing vagueness and imprecision in the data or the linguistic stakeholder evaluations. In this chapter an extended literature on fuzzy sets application of complex energy systems. The main issues emphasized in the literature review can be summarized as prediction and modelling the energy configuration conditions, interactions among the various critical design parameters, and solving power systems challenges under uncertainty. The fuzzy application on complex energy systems is presented for different energy types, such as bioenergy, wave energy, photovoltaic systems, hydrogen energy, nuclear energy, wind and thermal energy.