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CO line emission represents the most accessible and widely used tracer of themolecular interstellar medium. This renders the translation of observed COintensity into total H2 gas mass critical to understand star formation and theinterstellar medium in our Galaxy and beyond. We review the theoreticalunderpinning, techniques, and results of efforts to estimate this CO-to-H2"conversion factor," Xco, in different environments. In the Milky Way disk, werecommend a conversion factor Xco = 2x10^{20} cm^-2/(K km/s)^-1 with +/-30%uncertainty. Studies of other "normal galaxies" return similar values in MilkyWay-like disks, but with greater scatter and systematic uncertainty. Departuresfrom this Galactic conversion factor are both observed and expected. Dust-baseddeterminations, theoretical arguments, and scaling relations all suggest thatXco increases with decreasing metallicity, turning up sharply below metallicity~1/3-1/2 solar in a manner consistent with model predictions that identifyshielding as a key parameter. Based on spectral line modeling and dustobservations, Xco appears to drop in the central, bright regions of some butnot all galaxies, often coincident with regions of bright CO emission and highstellar surface density. This lower Xco is also present in the overwhelminglymolecular interstellar medium of starburst galaxies, where several lines ofevidence point to a lower CO-to-H2 conversion factor. At high redshift, directevidence regarding the conversion factor remains scarce; we review what isknown based on dynamical modeling and other arguments.

The CO-to-H2 Conversion Factor