If we allow for some randomness in some of the coefficients of a partial differential equation we often obtain a more realistic mathematical model of the situation where is called the two-dimensional stochastic differential equation or in more complicated cases, two-dimensional stochastic integral equation. Some situations where such equations appear and can be used are: Population dynamics, protein kinetics, genetics, experimental psychology, neuronal activity, option pricing, turbulent diffusion, radio-astronomy, helicopter rotor, satellite orbit stability, biological waste treatment,...