In this chapter we give an alternative, but equivalent description of open quantum systems using a stochastic extension of Heisenberg’s equations of motion. This is analogous to Langevin’s description of Brownian motion. However care must be taken in the quantum case to ensure that the commutation relations of the system observables obey equal time commutation relations. This requirement is satisfied by using operator noise terms rather than classical fluctuating forces. We show how this provides a useful way to compute correlation functions for the field emitted by an open system, a source, into its environment via the input-output relations for fields scattered from the source.