This chapter examines the weight build-up of an aircraft, the principal issues concerning aircraft performance comparisons, and how to compose a performance evaluation for a set of routes in the context of fleet planning. A word or two on how aircraft are certificated gives us a useful background when dealing with the aircraft weights and performance. There are two types of certification, the first being a domestic certification under the jurisdiction of the state of design and manufacture. The second type of certification involves the validation of the basic aircraft certification by the local, or national, authority of the country in which the aircraft will be operated. Although laws of aerodynamics dimension aircraft performance, operational weights are determined by design objectives. The chapter examines the forces that shape the payload-range envelope. This all-important diagram has little application in day-to-day aircraft operations, but is of considerable use in comparing aircraft capability in standard conditions.