This book shows how to use supercritical water (SCW) to synthesize nano- and micro- oxides, inorganic salts and metal particles and its recent advancement. Also polymer/biomass particles can be produced by using the method of precipitation of solutes from SCW. The particles can be used as catalysts for biomass conversions, materials in ceramics & electronic devices and composite materials. Particles are easily produced continuously in a flow reactor in short and long reaction times. Besides the synthesis process, the book also present studies of the properties of these materials. The size, size distribution, crystal growth & structure, and morphology of particles produced by supercritical water can be controlled by the feed concentrations, pH, pressures, temperatures, heating & cooling rates, organic modifications, reducing or oxidizing atmospheres, flow rates and reaction times.