Noctilucent clouds (NLC) occur close to 83 km altitude during summer at polar, high, and mid-latitudes. They are frequently visible to Earth-bound observers, provided the observers are on the night side of Earth and the clouds are still illuminated by the Sun. Under these conditions, NLC can become a quite impressive sight. NLC owe their existence to the extremely low temperatures (well below 150 K) which prevail during summer over a wide latitude band in the 82- to 90-km altitude region. For a major review of NLC science, the reader is referred to Gadsden and Schröder [1989].