  The decoherent histories approach is a natural medium in which to address
problems in quantum theory which involve time in a non-trivial way. This
article reviews the various attempts and difficulties involved in using the
decoherent histories approach to calculate the probability for crossing the
surface $x=0$ during a finite interval of time. The commonly encountered
difficulties in assigning crossing times arise here as difficulties in
satisfying the consistency (no-interference) condition. This can be overcome by
introducing an environment to produce decoherence, and probabilities exhibiting
the expected classical limit are obtained. The probabilities are, however,
dependent to some degree on the decohering environment. The results are
compared with a recently proposed irreversible detector model. A third method
is introduced, involving continuous quantum measurement theory. Some closely
related work on the interpretation of the wave function in quantum cosmology is
described.
