For the most part, gas phase models of the chemistry of dense molecular clouds
predict the abundances of simple species rather well. However, for larger molecules and even for
small systems rich in carbon these models often fail spectacularly. We present a brief review of the
basic assumptions and results of large scale modeling of the chemistry in dense molecular clouds.
Particular attention will be paid to the influence of the gas phase ratios of the major elements
in molecular clouds, and the likely role grains play in maintaining these ratios as clouds evolve
from initially diffuse objects to denser cores with associated stellar and planetary formation.
Recent spectral line surveys at centimeter and millimeter wavelengths along with selected
observations in the submillimeter have now produced an accurate "inventory" of the gas phase
elemental budgets in different types of molecular clouds, though gaps in our knowledge clearly
remain. The constraints these observations place on theoretical models of interstellar chemistry
can be used to gain insights into why the models fail, and show also which neglected processes
must be included in more complete analyses. Looking toward the future, truly protostellar regions
are only now becoming available for both experimental and theoretical study, and some of the
expected modifications of molecular cloud chemistry in these sources are therefore outlined
