To diminish a serious global energy crisis and global warming caused by the imbalance between the slow formation of fossil fuels and rapid consumption by human activities, the developing of the technology for quickly turning biomass and CO2 into fuels and chemicals is needed. Hydrothermal reactions, which can be recognized as an effective pathway in the carbon cycle, played an important role in forming petroleum, natural gas, and coal from organic wastes. If humans could simulate the natural phenomena of the formation of fossils, then, it should quickly turn biomass and CO2 into fuels and chemicals. This paper gives an overview of some recent studies on hydrothermal conversion of biomass such as carbohydrate, lignin, and glycerin into value-added chemicals. Some recent advance in hydrothermal reduction of CO2 is also presented.