  We propose an energy-driven stochastic master equation for the density matrix
as a dynamical model for quantum state reduction. In contrast, most previous
studies of state reduction have considered stochastic extensions of the
Schr\"odinger equation, and have introduced the density matrix as the
expectation of the random pure projection operator associated with the evolving
state vector. After working out properties of the reduction process we
construct a general solution to the energy-driven stochastic master equation.
The solution is obtained by the use of nonlinear filtering theory and takes the
form of a completely positive stochastic map.
