  Principles of monitoring non-Markovian open quantum systems are analyzed. We
use the field representation of the environment (Gardiner and Collet, 1985) for
the separation of its memory and detector part, respectively. We claim the
system-plus-memory compound becomes Markovian, the detector part is tractable
by standard Markovian monitoring. Because of non-Markovianity, only the mixed
state of the system can be predicted, the pure state of the system can be
retrodicted. We present the corresponding non-Markovian stochastic
Schr\"odinger equation.
