  CO clouds can be non-self-gravitating in high pressure environments, while
most should be strongly self-gravitating at low metallicities and ambient
pressures. In the LMC, which is HI-rich, GMC formation and destruction should
generally include molecule formation and destruction. In M51, which is CO-rich,
GMCs grow by coalescence. The Milky Way is between these two situations. In all
cases, large clouds form by accretion of gas and smaller clouds independently
of the presence of molecules. GMCs in the Milky Way are analogous to dust lanes
and spurs in other galaxies. The virial parameter alpha usually decreases
monotonically with increasing cloud mass in surveys, which implies that small
scale structure is formed by turbulence. Hierarchies of sequences with
decreasing alpha should be present in cloud complexes from sub-solar masses up
to the ambient Jeans mass (10^7 Msun).
