This chapter introduces the overall energy performance classification system. It is based on the concept of exogenous and endogenous influences on systems. The exo-endo classification concentrates attention on the users of energy-consuming systems and classifies the various features of a system according to its potential for modification. At a given level, exogenous features are external constraints on the energy-consuming systems, while endogenous features are aspects that are amenable to change. Endogenous factors for a nation are those aspects of a building about which a particular society or government may make regulations or offer incentives. The chapter introduces the idea that the applications required of a performance index should be the prime determinant of its form. It expands on the form of index used in a range of applications. An index of energy performance is a measuring tool which makes it possible to compare different levels of energy use in the provision of a particular type of service.