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 carbon chemistry in dense
molecular clouds. Particular attention will be paid to the influence of the gas phase C/O ratio in
molecular clouds, and the likely role grains play in maintaining this ratio 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
carbon budget in several 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, larger molecules are
especially difficult to study both experimentally and theoretically in such dense, cold regions, and
some new methods are therefore outlined which may ultimately push the detectability of small
carbon chains and rings to much heavier species