 It is often said that many physical phenomena can be explained as (more or less direct) consequences of symmetry principles or symmetry arguments. In the case of symmetry principles, the explanatory role of symmetries arises from their place in the hierarchy of the structure of physical theory, which in turn derives from their generality. As Wigner (1967, pp. 28ff) describes the hierarchy, symmetries are seen as properties of the laws. Symmetries may be used to explain (i) the form of the laws, and (ii) the occurrence (or non-occurrence) of certain events (this latter in a manner analogous to the way in which the laws explain why certain events occur and not others). In the case of symmetry arguments, we may, for example, appeal to Curie’s principle to explain the occurrence of certain phenomena on the basis of the symmetries (or asymmetries) of the situation, as discussed in section 3, above. Furthermore, insofar as explanatory power may be derived from unification, the unifying role of symmetries also results in an explanatory role.