Electron diffraction refers to changes in the direction of electron beams due to interactions with atoms. The resulting map of the directions of the electrons after they have interacted is called a diffraction pattern. It is similar to x-ray and neutron diffraction.

Electron diffraction occurs due to elastic scattering, when there is no change in the energy of the electrons during their interactions with atoms. The negatively charged electrons are scattered due to Coulomb forces when they interact with both the positively charged atomic core and the negatively charged electrons around the atoms; most of the interaction occurs quite close to the atoms, within about one Angstrom. In comparison, x-rays are scattered after interactions with the electron density while neutrons are scattered by the atomic nuclei through the strong nuclear force.

More rigorous details are given later; as a simple introduction, all matter can be thought of as waves, from small particles such as electrons up to chocolate brownies -- although it is impossible to measure any of the "wave-like" behavior of a brownie. Waves can move around objects and create interference patterns. A classic example is the Young's two-slit experiment shown in Figure 2, where a wave impinges upon two slits in the first of the two images. After going through the slits there are directions where the wave is stronger, ones where it is weaker -- the wave has been diffracted. If instead of two slits there are a number of small points then similar phenomena can occur as shown in the second image where the wave is coming in from the bottom right corner. This is comparable to diffraction of an electron wave where the small dots would be atoms. A map of the directions of the electron waves leaving the sample will show high intensity (white) for favored directions, such as the three prominent ones in the Young's slits experiment of Figure 2, while the other directions will be low intensity (dark). Often there will be an array of spots (preferred directions) as in Figure 1 and the other figures shown later.

The most common use of electron diffraction is in transmission electron microscopy (TEM) with thin samples of tens to at most a thousand atoms in thickness, that is 1 nanometer to 100 nanometers. Some details on methods for sample preparation of thin samples can be found in the book by Edington, within journal publications, in the unpublished literature and within the page transmission electron microscopy. There are many different ways to collect diffraction information in a TEM such as selected area, convergent beam, precession and 4D STEM as described below. There are also many other types of instruments. For instance, in scanning electron microscopy (SEM), electron backscatter diffraction is used to determine crystal orientation across the sample. Electron diffraction can also be used to characterize molecules using gas electron diffraction, surfaces using lower energy electrons, a technique called LEED, and by reflecting electrons off surfaces, a technique called RHEED.

There are also many levels of analysis and explanation of electron diffraction, elements of which are described later. These include:

1. The simplest approximation using the de Broglie wavelength for electrons, where only the geometry is considered and often Bragg's law is invoked.
2. The first level of more accuracy where it is approximated that the electrons are only scattered once, which is called kinematical diffraction.
3. More complete and accurate explanations where multiple scattering is included, what is called dynamical diffraction (e.g. refs).

Unlike x-ray diffraction and neutron diffraction where the simplest approximations are quite accurate, with electron diffraction this is not the case. Simple models give the geometry of the intensities in a diffraction pattern, but higher level ones are needed for many details and the intensities -- numbers matter.