  Equivalence principles played a central role in the development of general
relativity. Furthermore, they have provided operative procedures for testing
the validity of general relativity, or constraining competing theories of
gravitation. This has led to a flourishing of different, and inequivalent,
formulations of these principles, with the undesired consequence that often the
same name, "equivalence principle", is associated with statements having a
quite different physical meaning. In this paper we provide a precise
formulation of the several incarnations of the equivalence principle,
clarifying their uses and reciprocal relations. We also discuss their possible
role as selecting principles in the design and classification of viable
theories of gravitation.
