No quantum system is ever completely isolated. At the very least, it must be coupled to the outside world when we make measurements. In this chapter we discuss techniques to treat open quantum systems. We consider the system of interest as weakly coupled to an environment which remains close to a steady state. We first derive a master equation for the density operator of the open system in the Schrödinger or interaction picture. Using the quasi-probability representations for the density operator, the master equation may be converted to a c-number Fokker–Planck equation. We describe an alternative formulation in terms of quantum stochastic differential equations, replacing the Heisenberg equations of motion for closed quantum systems.