Waste management, energy demand, and pollutant emissions have been increasing global concerns. As a preferred waste management option, energy production from distributed renewable waste resources is gaining momentum worldwide, which can benefit the alleviation of the aforementioned issues, the reservation of land, and the development of local economy. Waste-to-energy technologies are mainly comprised of thermal processing (typically incineration, gasification, and pyrolysis) and biological processing (typically anaerobic digestion). Energy can be produced in a wide variety of forms, such as heat only, electricity (i.e., power) only, combined heat and power (CHP), CH4-rich biogas as transportation fuel, syngas, bio-diesel, ethanol, methanol, and gasoline. Incineration and anaerobic digestion are currently the most prevalent and commercialized methods with gasification and pyrolysis generally approaching the commercial stage. Gas emissions are the major concern for thermal technologies, which may involve NOx, SOx, volatile organic compounds (VOCs), vaporized heavy metals (e.g., Pb, Hg and Cd), polychlorinated dibenzodioxines and furans (PCDD/Fs), polycyclic aromatic hydrocarbons (PAH), HCl, HF, NH3, and others. The leading countries in waste-to-energy (WtE) activities such as EU nations and US are implementing stricter emission standards than developing countries like China. Ammonia is one of the major pollutants released from anaerobic digestion of renewable wastes. To achieve sustainable production of waste-to-energy systems, some uncertainty factors are needed to be taken into account. In this chapter, factors of sustainable supply of materials, technological aspects of waste-to-energy systems, policy instruments and incentives, social and political factors, and the lack of accessible data have been disscussed.