  The CO-H2 conversion factor in galaxies is typically described as bimodal:
one value for discs and quiescent regions, and another (lower) value for
mergers and starbursts. In this proceeding, I will describe both empirical
observational evidence that the conversion factor varies with physical
environment, as well as a theoretical model which aims to understand the
physical processes which drive these variations. I present a functional form
for Xco which can be applied to observations ranging in scale from ~70 pc to
galaxy-wide scales, and show the consequences of the application of this model
to the Kennicutt-Schmidt star formation law.
