This paper discusses the structural interface on FPSOs between the hull structure and the topsides modules. It identifies the most common topsides foundation concepts applied on FPSOs, and discusses the consequences of each configuration for the layout of the unit, the design of the hull structure and the topsides. The information needed by the hull designer and the topside designer is identified. Moreover, the differences between shipbuilding and offshore construction design practices are discussed, and it is identified where and how these fall short for FPSO purposes. Topics that are addressed are overall safety, operational aspects, such as tank entry and mechanical handling, and the design specifications for the hull and the topsides modules. In order to control the schedule and costs of FPSO projects, fabrication of the hull and topsides should be allowed without impractical or unduly strict specifications imposed on the shipyard or the topsides fabricator. At the same time the traditional design specifications for hull and topsides design may fall short to cover the functional needs for FPSO service.