Flexible studio talent who began his career as a critic, becoming a scenarist in 1926 and a director in 1932. Leaving Germany the year Hitler took power, Koster made several films in Europe before going to Hollywood. His US debut with "Three Smart Girls" (1937), the first in a series of Deanna Durbin vehicles, was a resounding success and helped bolster the straitened Universal studios.

Koster is perhaps best known for the charming "Harvey" (1950), in which James Stewart played opposite an invisible six-foot rabbit; he also directed the rather stodgy but well-remembered "The Robe" (1953), notably primarily as the first film to be shot in CinemaScope.