Thorium (Th) is the first element in the Actinide series having atomic number 90. It is a radioactive element. Thorium oxides can be used as nuclear fuels, although this application is quite limited compared with uranium oxide materials. In contrast to the well-known properties of bulk thorium oxides, thorium oxide molecules are much less understood. This book discusses the chemical properties, uses and environmental effects of thorium. Topics discussed include the products of laser ablated thorium atom reactions with small molecules; soil profile thorium and uranium concentration distributions in southeastern Missouri soils; influences of coal-fired power plants on the thorium levels in soils and on radioactive hazards for the population; behavior of Th-isotopes along the pigment TiO2 industrial production process; some features of the Th presence in the poços de Caldas alkaline massif, Brazil; geological and geophysical implications for western Australia; hermophysical and thermodynamic properties of oxygen-containing compounds of thorium; and unique role of thorium in closed (Th-U-Pu)-fuel cycle due to the involvement of hybrid "fusion-fission" reactors to generate advanced (231Pa+232U+233U)-fuel.