Solution plasma (SP) is non-equilibrium plasma generated in the liquid phase. Plasma contains electrons of which temperature is as high as 104 K. Such high-energy electrons trigger electron impact reactions such as excitation, ionization, and dissociation of molecules and produce active species such as ions, atoms, and radicals. These chemically active species produced in the plasma are supplied to the liquid phase in the SP process (SPP). Chemical reactions at a plasma-liquid interface occur much faster than conventional solution chemistry. Another important feature of the SP is the fact that the plasma is non-equilibrium. It should be noted what the “non-equilibrium” means because this is the most important feature of the SP. In the case of equilibrium plasma, the temperature of the medium may be as high as the electron temperature of 103–104 K. Such plasma is called thermal plasma, and its use is limited to high-temperature applications such as welding, machining, and waste treatments. In the case of non-equilibrium plasma, by contrast, the temperature of gas and/or liquid is as low as the room temperature even though the electron temperature is as high as 104 K. This feature of SPP results in a situation in which active species can participate in the chemistry of solutions without having an excessive thermal effect. This chapter describes various examples of the use of SPPs for materials synthesis.