This paper describes a formalism and implementation for the interpretation and generation of sentences containing context-dependent constructs like determiners, pronouns, focus, and ellipsis. A variant of quasi-logical form is used as an under specified meaning representation, related to resolved logical forms via conditional equivalences. These equivalences define the interpretation of contextually dependent constructs with respect to a given context. Higher-order unification and abduction are used in relating expressions to contexts. The conditional equivalences can be used unchanged in both the interpretation and the generation direction.