Solution Plasma (SP) is an electrical discharge at atmospheric pressure in the gas formed from a surrounding liquid. Water, aqueous solution, organic compound, organic solution, suspension, liquid nitrogen, supercritical fluids, or ionic liquids can be the solution in the SP. The SP is a cold and non-local Thermodynamic Equilibrium plasma where the electron temperature is much greater than the gas temperature. The SP operates in a glow discharge regime. The processes inside SP are named Solution Plasma Processes (SPP). SPP combines the physics of atmospheric pressure discharges, the thermodynamic properties of liquids, and the physical and chemical reactions at the interfaces: plasma gas-phase and solution, plasma gas-phase and electrodes surface, and solution-electrodes interface. Four species of particles exist in a plasma gas phase: electrons, ions, neutrals, and radicals. The species densities and temperatures characterize the plasma. This chapter is focused on the formation of the SP and the diagnostics methods using electrical and optical procedures. The electrical characteristics of the SP mean the measurement of the current, voltage, and electron temperature. The optical diagnostics methods like emission and absorption spectroscopy measure the electron and particle densities in the SP gas and liquid phases.