With the introduction of the Payload Assist Module (PAM) the McDonnell Douglas Astronautics Company pioneered the development of the first commercial US orbital transfer vehicle. The PAM program was initiated in 1976 to provide spacecraft developers with an orderly transition from the Delta expendable launch vehicle to the space shuttle. With the PAM system, medium-sized payloads designed to be placed in orbit from the Delta vehicle could, without any redesign, be launched from the space shuttle when it became operational. During the late 1970s and early 1980s, an evolutionary increase in the weight of spacecraft to be launched and the introduction of the operational space shuttle system produced a need for new, economical transfer vehicle systems. To meet this need, and also take advantage of the larger payload volume available in the shuttle orbiter, several companies initiated the commercial development of additional upper-stage systems. This paper provides an overview of upper-stage systems that are currently operational, under development, or in the planning stages. Each system is described, performance comparisons are made, the operational status is identified, and a discussion of what might be anticipated for commercial transfer vehicles in the early 1990s is provided.