We present a formulation of non-Markovian quantum trajectories for open sys
tems from a measurement theory perspective. In our treatment there are thre
e distinct ways in which non-Markovian behavior can arise: a mode-dependent
coupling between the hath (reservoir) and system, a dispersive bath, and s
pectral detection of the output into the bath. In the first two cases the n
on-Markovian behavior is intrinsic to the interaction; in the third case th
e non-Markovian behavior arises from the method of detection. We focus in d
etail on the trajectories that simulate real-time spectral detection of the
light emitted from a localized system. In this case the non-Markovian beha
vior arises from the uncertainty in the time of emission of particles that
are later detected. The results of computer simulations of the spectral det
ection of the spontaneous emission from a strongly driven two-level atom ar
e presented. [S1050-2947(99)01403-1].