The dynamical theory of electron diffraction is applied to the interpretation of electron microscopic images of moire patterns. Two cases often observed are treated. One is the case where two plate-shaped crystals are superposed closely without a vacuum layer between them and another is the case where two crystals are superposed with a vacuum layer between them. Resolved lattice images of two superposed crystals are also interpreted. The intensity profiles of the images vary with the thicknesses of the crystals and vacuum layer and with the deviation from the Bragg angle. The shifts of the fringes and anomalies of the contrast which are expected from the present theory were observed in the electron microscopic images of moire patterns of cupric sulphide, palladium-gold and platinum-phthalocyanine. The relation between moire patterns and crystal structure is also discussed.