  We present virial models for the global evolution of giant molecular clouds.
Focusing on the presence of an accretion flow, and accounting for the amount of
mass, momentum, and energy supplied by accretion and star formation feedback,
we are able to follow the growth, evolution, and dispersal of individual giant
molecular clouds. Our model clouds reproduce the scaling relations observed in
both galactic and extragalactic clouds. We find that accretion and star
formation contribute contribute roughly equal amounts of turbulent kinetic
energy over the lifetime of the cloud. Clouds attain virial equilibrium and
grow in such a way as to maintain roughly constant surface densities, with
typical surface densities of order 50 - 200 Msun pc^-2, in good agreement with
observations of giant molecular clouds in the Milky Way and nearby external
galaxies. We find that as clouds grow, their velocity dispersion and radius
must also increase, implying that the linewidth-size relation constitutes an
age sequence. Lastly, we compare our models to observations of giant molecular
clouds and associated young star clusters in the LMC and find good agreement
between our model clouds and the observed relationship between H ii regions,
young star clusters, and giant molecular clouds.
