In Chap. 16 we developed the equation of motion for a system as it evolved under the influence of an unobserved (reservoir) system. We used the reduced density matrix concept and worked in an interaction picture. In this chapter we also consider the system-reservoir problem, but work with quantum oper­ ator equations of motion (see Sec. 7-4). Here too we eliminate the reservoir variables and find that the operator equations of motion for the system ac­ quire damping terms and “noise” operators which produce fluctuations. The resulting equations resemble the classical Langevin equations which describe, for example, the Brownian motion of a particle suspended in a liquid or the behavior of a current as it passes through a resistor.