Since the 1970s undulators are used in storage rings and free-electron lasers as sources of intense radiation. This tutorial gives an elementary introduction and describes the principles as well as electromagnetic, superconducting and permanent magnet technologies, which are used in practice. Special emphasis is put on permanent magnet technology, which is most developed and used in most practical applications. An overview illustrated by many examples of the state of the art is given. CERN Yellow Reports: School Proceedings, Vol 1 (2018): Proceedings of the CAS-CERN Accelerator School on Free Electron Lasers and Energy Recovery Linacs