  We have developed a code that models the formation, destruction, radiative
transfer, and vibrational/rotational excitation of H_2 in a detailed fashion.
We discuss how such codes, together with FUSE observations of H_2 in diffuse
and translucent lines of sight, may be used to infer various physical
parameters. We illustrate the effects of changes in the major physical
parameters (UV radiation field, gas density, metallicity), and we point out the
extent to which changes in one parameter may be mirrored by changes in another.
We provide an analytic formula for the molecular fraction, f_H2, as a function
of cloud column density, radiation field, and grain formation rate of H_2. Some
diffuse and translucent lines of sight may be concatenations of multiple
distinct clouds viewed together. Such situations can give rise to observables
that agree with the data, complicating the problem of uniquely identifying one
set of physical parameters with a line of sight. Finally, we illustrate the
application of our code to an ensemble of data, such as the FUSE survey of H_2
in the Large and Small Magellanic Clouds (LMC/SMC), in order to constrain the
elevated UV radiation field intensity and reduced grain formation rate of H_2
in those low- metallicity environments.
