  We present a stochastic path integral formalism for continuous quantum
measurement that enables the analysis of rare events using action methods. By
doubling the quantum state space to a canonical phase space, we can write the
joint probability density function of measurement outcomes and quantum state
trajectories as a phase space path integral. Extremizing this action produces
the most-likely paths with boundary conditions defined by preselected and
postselected states as solutions to a set of ordinary differential equations.
As an application, we analyze continuous qubit measurement in detail and
examine the structure of a quantum jump in the Zeno measurement regime.
