The Space Transportation System (STS) was initiated to provide the capability for reliable, low-cost delivery of commercial, scientific, and military payloads into their respective operational orbits.(1)* Since each payload is unique in both design and objective, manifesting two or more payloads together on the same flight can be a very involved process. The mixing process consists of analyses in the technical areas of Utilization Planning and Flight Planning. These analyses determine the compatibility of the payloads with each other and with the Shuttle System. This paper presents an overview of the payload mixing process, discusses the two major areas involved in payload mixing, and presents a simplified example.