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We have compared the properties of GMCs in 5 galaxies, four of which have been surveyed in their entirety: the LMC, the SMC, M33, IC 10. M31 was observed over a very limited area. The interstellar medium of all five galaxies is dominated by the atomic phase.

  1. The GMCs do not, in general, show any relationship to the stellar content of the galaxies except for the O stars born in the GMCs.

  2. There is a very good correlation between the locations of the GMCs and filaments of H I. Many filaments contain little of no molecular gas even though they have similar surface densities compared to those that are rich in GMCs. This suggests that clouds form from the H I rather than vice-versa.

  3. There appears to be a clear evolutionary trend going from filament formation --> molecule formation --> GMC formation. It is not clear however, whether the condensations that form GMCs are first formed in the atomic filaments, or only after the molecules have formed.

  4. We derive XCO for all of the galaxies assuming that the GMCs are virialized. Although there is some variation, a value of XCO = 4 × 1020 cm-2 (K km s-1)-1 is a representative value to within about 50% except for the SMC, which has a value more than 3 times higher. There is no clear trend of XCO with metallicity.

  5. The discrepancy between the virial value and the value determined from gamma-ray observations in the Milky Way suggests that the GMCs are not virialized, if the gamma-ray value is applicable to other galaxies in the Local Group. In that case, a value of XCO = 2 × 1020 cm-2 (K km s-1)-1 may be more appropriate.

  6. The GMCs in our sample appear to satisfy the line width-size relation for the Milky Way, but with an offset in the constant of proportionality. This offset may be due, at least in part, to the different data analysis techniques for the MW and extragalactic data sets. For a given line width, the extragalactic clouds appear to be about 50 % larger. Despite the systematic offset, there are small but significant differences in the line width-size relationship among GMCs in different galaxies.

  7. The GMCs within a particular galaxy have a roughly constant surface density. If the value of XCO we derive for each galaxy is applied, the surface densities of the sample as a whole, have a scatter of less than a factor of two.

  8. The mass spectra for the GMCs in all of the galaxies can be characterized as a power law with a slope of ~ -1.7, with the exception of M33, which has a slope of -2.5.

  9. The ratio of H2 to H I on a pixel-by-pixel basis in galaxies appears to be determined by the hydrostatic pressure in the disk.

  10. About 1/4 - 1/3 of the GMCs in the LMC and M33 appear to be devoid of high-mass star formation.

  11. The association of stars and H2 regions in the LMC suggests a lifetime for the GMCs of about 27 Myr, with a quiescent phase that is about 25% of the age of the GMCs. In M33, a lifetime of ~ 20 Myr is measured. For GMCs in these galaxies we estimate that typical lifetimes are roughly 20-30 Myr. Both lifetimes are uncertain by about 50%.

Acknowledgments. This work is partially supported by US National Science Foundation under grants AST-0228963 and AST-0502605, a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan (No. 15071203), and from JSPS (No. 14102003). The NANTEN project is based on a mutual agreement between Nagoya University and the Carnegie Institution of Washington (CIW). We greatly appreciate the hospitality of all the staff members of the Las Campanas Observatory of CIW. We are thankful to many Japanese public donors and companies who contributed to the realization of the project. We would like to acknowledge Drs. L. Stavely-Smith and M. Filipovic for the kind use of their radio continuum data prior to publication.

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