The purpose of this short lecture is to explain why defects are imaged in the electron microscope, and to emphasize that a good understanding of the theory can lead to important informations about the defects. First, the different notions and parameters describing the diffraction of electrons by a perfect foil are introduced and discussed to some extent : wavelength, Bragg reflection, Bragg angle, dispersion surface, extinction distance, excitation error, absorption length, bright field, dark field, extinction contours, column approximation. Next, one considers planar interfaces separating two crystal parts, like stacking faults, antiphase boundaries, domain boundaries. It is shown that the amplitudes of the transmitted and scattered beam can be calculated by considering the transmitted and scattered waves emerging from the first part again as incident on the second part of the crystal foil. The image is a fringe pattern, the fringe spacing depending on the crystal and the active reflection, and not on the type of interface. Some properties of the image are reviewed, in particular those which enable to differentiate intrinsic and extrinsic stacking faults in f. c. c metals, and the property which enables to distinguish stacking faults and domain boundaries. In the last part of the lecture, the contrast image is discussed of defects like dislocations, small loops, precipitation, voids, clusters a. s. o. The image is in this case a consequence of the elastic strains which deform continuously the reflecting lattice planes. Some qualitative properties of dislocations are briefly discussed, which can be used to determine the Burgers vector.