The twentieth century saw more change in organ styles happen more quickly than any other time in organ history. Furthermore, unlike earlier periods, the twentieth century witnessed different trends existing and developing side by side. For example, nineteenth-century traditions continued in the early 1900s alongside the development of the progressive symphonic organ. Shortly after the middle of the century, in some countries a newly built Orgelbewegung instrument with mechanical action could be found near a new organ with electro-pneumatic key action designed in an eclectic style. In the century's final decade, the eclectic organ predominated, but at the same time one could find new instruments built along strictly historical lines, as well as neo-symphonic organs inspired by trends from the early twentieth century. This survey first examines the seven broadly conceived building styles that inform the twentieth-century organ, then considers the same period in a review by geography.