Marcussen & Søn, also known as Marcussen and previously as Marcussen & Reuter, is a Danish firm of pipe organ builders. They were one of the first firms to go back to classical organ-building techniques, and have been producing mechanical-action organs since 1930. Aside from their many instruments in Denmark, they have built organs in northern Germany, Sweden, Finland, the Netherlands, Great Britain, South Africa, Japan, and the United States.