The available data on the microwave spectrum of water vapor are critically reviewed for information applicable to radio astronomy. Molecular data such as rotational constants, centrifugal distortion constants, hyperfine coupling parameters, and dipole moments are tabulated. A detailed centrifugal distortion calculation has been carried out for the most abundant isotopic form of this molecule H216O, as well as for H218O and HD16O. Transitions have been predicted and tabulated for the frequency range 1 MHz to 800 GHz. All predicted transitions include 95 percent confidence limits; estimated error limits have been reported for all measured transitions. Observed transitions of H217O are also listed.