The available data on the microwave spectrum of isocyanic acid are critically reviewed for information applicable to radio astronomy. Molecular data such as rotational constants, centrifugal distortion parameters, dipole moments, hyperfine coupling constants, and structural parameters are tabulated. Detailed centrifugal distortion calculations have been carried out for all isotopic forms of this molecule, including DNCO. Transitions have been predicted for the parent molecule for the frequency range 160 MHz−300 GHz. All predicted transitions include error limits. The quoted uncertainties represent one standard deviation. A 95 percent confidence limit is obtained by using approximately twice the calculated standard deviation. Estimated error limits for the measured transitions are discussed. References are given for all data included.