The microwave spectrum of acetaldehyde is critically reviewed and supplemented with spectral frequency calculations derived from the rotation‐internal rotation analysis. A simultaneous analysis of the torsional ground state,vt=0, and first and second torsionally excited states,vt=1 and 2, was carried out. The primary objective of this review is to provide radio astronomers with complete spectral coverage over the range of 900 MHz to 500 GHz for the ground state and covering rotational quantum number,J, from 0 to 26. ©1996 American Institute of Physics and American Chemical Society.