The demand for renewable energy is increasing due to the shortage of energy supply and anticipated global warming due to greenhouse gas emissions from fossil energy plants. In this regard, the significance of WtE (Waste to Energy) in terms of both technology and policy has been significant in recent years. In fact, WtE technology that produces energy from byproducts and garbage already has a potential to cover around 80% of total renewable energy generated in Korea. The waste and biomass with inclusion of organic portions are chemically akin to each other, and the technologies that utilize them have shown some similarity. Therefore WtE technology is an attractive option that can handle both waste and biomass in conjunction, while heat or electricity generation from the conventional technologies such as waste incineration has been already well implemented in advanced countries. In order to achieve higher efficiency and better performance in the energy generated from the wastes, several new ideas and innovative thermal technologies have been developed and introduced: pyrolysis, gasification, and liquefaction with less green-house gas emissions.