The spectrum of the NaK molecule has been studied in absorption and in magnetic rotation throughout the visible and photographic infrared regions. By means of magnetic rotation the upper vibrational levels of the orange system have been followed nearly to dissociation, permitting accurate determinations of the energies of dissociation of four states of the molecule, that of the ground state being 0.62\ifmmode\pm\else\textpm\fi{}0.03 volt. Two new band systems, in the green and infrared, have been found and analyzed. The fourth theoretically expected system has not been found and is presumably covered by strong ${\mathrm{Na}}_{2}$ and ${\mathrm{K}}_{2}$ systems. The magnetic rotation spectra in the green and red have no simple relation to the absorption spectra and are probably to be interpreted as due to perturbations, like the red magnetic rotation spectrum of ${\mathrm{Na}}_{2}$.