Abstract: A window sleeve arrangement for positioning a window unit in an opening through an enclosure wall has an interior surface for receiving the window unit and an outside surface for sliding into the opening through the enclosure wall. A first exterior flange engages the exterior surface of the enclosure wall, while a second exterior flange, spaced from the first exterior flange provides a gap for receiving sheathing on the outside of the building. Disposed within the window sleeve arrangement is an interior inwardly projecting flange which serves as a window stop to position the window adjacent the exterior wall of the building enclosure. The window unit is held within the window sleeve by a snap-in-place molding which engages the window unit. Optionally, interior decorative molding is coupled to the sleeve to cover a portion of the interior wall adjacent to the window sleeve.