Water is an imperative component of hydrothermal systems and both temperature and pressure have a great influence on the resulting properties of water. When water attains supercritical state, its surface tension approaches nearly zero. At this point, the distinction between gas and liquid phase breaks down and water effectively occupies the pores of the material and facilitates the hydrothermal process [1]. Hydrothermal processing can be defined as “any homogeneous (nanoparticles) or heterogeneous (bulk materials) reaction in the presence of aqueous solvents or mineralizers under high-pressure (above 1 atmospheric) and temperature (above a room temperature) conditions to dissolve and recrystallize (recover) materials that are relatively insoluble under ordinary conditions” [1]. Hydrothermal synthesis of materials indeed has a long-standing history. Schafthaul was the first person to use hydrothermal treatment to prepare fine particles of quartz in a Papin’s digester during 1845 [2]. In the early 1900s, more than 150 mineral species, including diamond, were synthesized by hydrothermal methods [3]. Since the 1940s, the early stage of hydrothermal research was conducted 508by several groups in the United States, Europe, and Japan. They mostly focused on crystal growth of artificial Zeolite and Quartz as indicated in a review by Somiya [4]. During the years between 1950s and 1970s, hydrothermal processes have become a facile method mostly used in the area of geoscience [5-7]. However, ever since the late 1970s, material scientists adopted hydrothermal processes to prepare various compounds with controlled size, shape, and composition. During the late 1980s, solvothermal reaction has been defined as a high-temperature reaction in a closed system in the presence of a solvent (generally nonaqueous) [8a,b]. A solvothermal process can also be termed as glycothermal, ammonothermal, carbonathermal, lyothermal, alchothermal, or hydrothermal depending on the nature of the used solvent. Recently, nonaqueous solution processes have also been developed, which generates a small amount of H 2 O during the process. This small amount of water then becomes a major reactant in the actual solution process. Hydrothermal and solvothermal process might also relate to supercritical fluid process, molten salt, and/or ionic liquid process [1,9]. Hydrothermal research nucleated in Japan, the United States, and other parts of the world and this opened a new avenue for hydrothermal research in the area of materials research [4-7,10-17]. The evolutionary trends of hydrothermal processing of various materials are given in Table 18.1 [9]. Yoshimura et al. have proposed a novel concept of soft solution process (SSP), for fabricating ceramic and other materials as a more effective and environmental-friendly approach and the end products have no disparity with hydrothermal or other synthetic routes [18-21]. Although this term has a broader meaning, it covers a part of the hydrothermal research and refers mainly to any solution processing at or near the ambient conditions. Hydrothermal process has been widely investigated and used for the synthesis of a wide range of materials, especially various metal oxides and nonoxides, such as diamond, carbon, selenides, tellurides, sulfides, fluorides, nitrides, and arsenides. Hydrothermal technologies have evolved as a very powerful tool in materials processing because it is versatile and environmentally benign. Hydrothermal treatment is, thus, highly suitable for advanced materials synthesis starting from bulk single crystals to fine and ultrafine crystals, including nanocrystals or nanoparticles with a controlled size and morphology [9,22]. Recently, hydrothermal processes have also emerged as a promising technique for the preparation of carbon material from various sources and importantly carbides, organic compounds, and biomass as described subsequently in more detail.509. © 2017 by Taylor & Francis Group, LLC.