3D electron diffraction 3D ED is a promising method for characterizing the structure of materials from nano- and micron-sized crystals. The method can be applied to crystals that are too small for single crystal X-ray diffraction. Rapid developments in specimen preparation, data collection, data processing, new software and hardware make it possible to obtain accurate crystal structure models by 3D ED in a few hours. However, comparing to X-ray diffraction methods, 3D ED is still in its infancy. New methods are actively being developed in this small but vibrant research field. This article introduces the history of the 3D ED development, the theoretical background of electron diffraction methods, the latest experimental protocol of continuous rotation 3D ED, and the specimen preparation for electron diffraction experiments. A number of selected examples of 3D ED applications are also covered in the article. Finally, the future perspective of 3D ED is discussed.