  Quantum trajectory theory, developed largely in the quantum optics community
to describe open quantum systems subjected to continuous monitoring, has
applications in many areas of quantum physics. In this paper I present a simple
model, using two-level quantum systems (q-bits), to illustrate the essential
physics of quantum trajectories and how different monitoring schemes correspond
to different ``unravelings'' of a mixed state master equation. I also comment
briefly on the relationship of the theory to the Consistent Histories formalism
and to spontaneous collapse models.
