The benefits and problems of BEMSs have been discussed in Chapter 1, but in this chapter the nature and details of the BEMS will be examined. First the outstation will be considered. This is the BEMS unit closest to the sensors and actuators and is often situated in the plant room. Other terms for the outstation are field processing unit (FPU), data gathering panel (DGP), distributed processing unit (DPU), field interface device (FID) and substation [1, 2]. The term FPU implies limited local processing power, whereas the DPU is more like the modern outstation considered in this book.