To treat the laser in a realistic way, we need the density matrix concepts and formalism given in Chap. 7. This material makes it possible to incorporate dipole phase decays due, for example, to collisions and to treat multimode electric fields in a straightforward way. Nevertheless, it is gratifying to find that we can already determine the field intensity for a simplified model of the ammonia beam maser, the first device of the maser-laser genre. The maser was originally conceived in 1951 by Townes, who noted his thoughts on the back of an envelope during a period of contemplation on a bench in Franklin Park, Washington, DC.T The device was realized experimentally by Gordon, Zeiger, and Townes (1954). In this chapter, we treat the maser along the lines given by Lamb (1960). The analysis is not complicated by atomic (molecular) lifetimes, because they are long compared to the Rabi flopping time, which is the optimal transit time for molecules in the maser cavity. Furthermore, by considering single-mode, steady-state operation, we can assume by energy conservation (1) that the energy given up by the atoms due to stimulated emission is gained by the field, and (2) that this energy flow balances the cavity losses.