In this chapter we use the Langevin (Brownian motion) method of Chap. 19 to once again treat quantum mechanically both the field and the active atoms in a laser. The present treatment comprises a Heisenberg picture approach to the problem, as compared to the approach in Chap. 17, which represents the Schrodinger picture. The two methods are in complete agreement (as they should be!), a point discussed in some detail by Lax (1968). As before, the atoms are considered to comprise systems which interact with independent pumping and dissipation reservoirs, and the laser field is regarded as another system with reservoirs. The atom and field systems are then coupled by an electric-dipole perturbation energy, as depicted schematically in Fig. 20-1.