Although pronouns, proper names and definite descriptions are some of the most basic of linguistic expressions, it is only in the last thirty years that the rules that govern their behavior have been described in formal detail. For example, in "He likes the director's mother" the pronoun "he" and the description "the director" cannot refer to the same individual (Condition C); but no such constraint holds in "His mother likes the director". While sophisticated rules have been developed in recent linguistics to describe these constraints, their very existence has never been satisfactorily explained. The present project seeks to develop a novel account in which these constraints are the by-product of the very procedure by which meaning is computed. The basic observation is that a general principle of economy prohibits definite descriptions from containing more material than is necessary to single out their intended referent - which explains why it is odd to speak of "John's blond father", since "blond" is semantically idle (since one generally has but one father). In simple cases, this economy principle relates a description to its extra-linguistic context. It is suggested, however, that meaning is computed by literally extending the extra-linguistic context with the denotation of the pronouns, proper names and descriptions that are found within a sentence. When the same economy principle is applied to this extended context, the classical examples of the constraints (especially Condition C) can be explained, as well as some new data that had not hitherto been described. The project will thus add to the depth of existing theories and offer a more fine-grained description of the relevant facts, which will be studied by comparing data from English and from Romance and Slavic languages.