  We present the first comparative study of extragalactic GMCs using complete
data sets for entire galaxies and a uniform set of reduction and analysis
techniques. We present results based on CO observations for the LMC, SMC, M33,
M31, IC10 and the nucleus of M64, and make comparisons with archival Milky Way
observations. Our sample includes large spirals and dwarf irregulars with
metallicities that vary by an order of magnitude. GMCs in HI rich galaxies are
seen to be well-correlated with HI filaments that pervade the galactic disks,
suggesting that they form from pre-existing HI structures. Virial estimates of
the ratio of CO line strength to H2 column density, XCO, suggests that a value
of 4 x 10^20 cm^-2 (K km/s)^-1 is a good value to use in most galaxies (except
the SMC) if the GMCs are virialized. However, if the clouds are only marginally
self-gravitating, as appears to be the case judging from their appearance, half
the virial value may be more appropriate. There is no clear trend of XCO with
metallicity. The clouds within a galaxy are shown to have the about the same H2
surface density and differences between galaxies seem to be no more than a
factor of ~2. We show that hydrostatic pressure appears to be the main factor
in determining what fraction of atomic gas is turned into molecules. In the
high-pressure regions often found in galactic centers, the observed properties
of GMCs appear to be different from those in the found in the Local Group. From
the association of tracers of star formation with GMCs in the LMC, we find that
about 1/4 of the GMCs exhibit no evidence of star formation and we estimate
that the lifetime of a typical GMC in these galaxies is 20--30 Myr.
