  We discuss recent models of chemical evolution in the developing and
collapsing protostellar envelopes associated with low-mass star formation. In
particular, the effects of depletion of gas-phase molecules onto grain surfaces
is considered. We show that during the middle to late evolutionary stages,
prior to the formation of a protostar, various species selectively deplete from
the gas phase. The principal pattern of selective depletions is the depletion
of sulfur-bearing molecules relative to nitrogen-bearing species: NH3 and N2H+.
This pattern is shown to be insensitive to the details of the dynamics and
marginally sensitive to whether the grain mantle is dominated by polar or
non-polar molecules. Based on these results we suggest that molecular ions are
good tracers of collapsing envelopes. The effects of coupling chemistry and
dynamics on the resulting physical evolution are also examined. Particular
attention is paid to comparisons between models and observations.
