An account is given of the electron-optical principles of simple and high-resolution electron diffraction cameras, and of the combined electron microscope and diffraction camera. Electron diffraction patterns can be used to identify materials, to determine the size, shape and orientation of crystals and to determine the orientation of large molecular groups in crystals. The patterns can also be used to study the stresses which occur naturally in many materials when the crystallites are very small. Some results of the dynamical theory of electron diffraction are stated and future applications are outlined.