We provide a scheme for determining which global variables are involved when an expression is evaluated in a language with higher order constructs and imperative features. The heart of our scheme is a mechanism for computing the support of an expression, i.e. the set of global variables involved in its evaluation. This computation requires knowledge of all the aliases of an expression. The inference schemes are presented as abstract semantic interpretations. We prove the soundness of our estimates by establishing a correspondence between the abstract semantics and the standard semantics of the programming language.