The equipment carried in a rocket or satellite to perform a set of specific experiments is generally only a small fraction of the total, the remainder providing ancillary services to the scientific payload, such as data storage and transmission, power generation and distribution, command reception, tracking facilities, stabilization and so on. The experiments must of course operate within the limitations of what it is technically feasible at any time for these services to provide. Evidently an appreciation of the current state of development of the various components of the spacecraft system is essential right from the start of the design of an experiment. The purpose of this chapter is to discuss these limitations as they exist at the time of writing, and to project the probable developments of the next few years. Figure 1 shows a block diagram of the entire system for a small, relatively simple satellite, Ariel I (NASA report, 1963). It will be useful to bear this in mind as an illustration for the remainder of the chapter, although techniques are widely divergent between different satellites, and none can be considered typical.