The available data on the microwave spectrum of methylenimine are critically reviewed for information applicable to radio astronomy. Molecular data such as rotational constants, centrifugal distortion parameters, hyperfine coupling constants, and dipole moments are tabulated. A detailed centrifugal distortion calculation has been carried out for the most abundant isotopic form of this molecule, H212C14NH. Transitions have been predicted and tabulated for the frequency range 100 MHz to 300 GHz. All predicted transitions include 95 percent confidence limits; error limits have been reported for all measured transitions.