This chapter summarizes the synthesis of various types of nanoparticles as well as surface modifications of nanomaterials using hydrothermal and solvothermal methods. First, the definition, history, instrumentation, and mechanism of hydrothermal and solvothermal methods as well as the important parameters affecting the nucleation and crystal growth of nanomaterials are briefly introduced. Then, the specific hydrothermal and solvothermal methods used to grow oxides; Groups II–VI, III–V, and IV; transitional metals; and metal-organic framework nanoparticles are summarized. Finally, the hydrothermal and solvothermal strategies used for the surface modification of nanomaterials are discussed.