It is shown that image upconversion using optically nonlinear materials is equivalent to a conventional hologram constructed at the signal wavelength and readout at the upconverted wavelength. As such, it should be a truly three‐dimensional, high‐resolution process. For the particular case of equal signal and pump wavelengths, the upconverted image will have only half the angular extent of the original object wave. The image's transverse dimensions remain unchanged while its location relative to the frequency doubler and its longitudinal dimensions double in size. It is also shown how the phase matching condition follows from the vector condition for the reconstruction of a thick hologram.