This dissertation presents a new perspective on the identity condition underpinning ellipsis in natural language. It argues that the condition is irreducibly syntactic—at least in part—but the way this syntactic component works is different than previously thought. First, instead of simple identity of structures/features, the condition relies on non-distinctness. For example, a privative feature present in the antecedent but not in the ellipsis site (or vice-versa) does not constitute a violation of identity. Nor does a functional projection present in one but not the other. Second, the identity condition includes a component that pertains to √ROOTs. Unlike the component requiring featural non-distinctness, √ROOTs in the ellipsis site and the antecedent must be strictly identical. After providing an overview of the core research questions surrounding ellipsis, the dissertation builds its initial case in chapter 2 on the basis of novel data from Kaqchikel (Mayan). In contrast to the pattern familiar from languages like English, Kaqchikel bans certain voice mismatches under sluicing, but allows others. To account for that, I argue that clauses in the Agent Focus voice—which can mismatch with active and passive clauses—lack the VoiceP layer. The proposed identity condition which relies on non-distinctness captures this newly-established pattern. The empirical scope is expanded in chapter 3, where I consider mismatches above VoiceP in several languages. I show that the proposed identity condition can account for the observed generalizations regarding tense, polarity, illocution, and modality mismatches, which remain unexplained under other proposals. Chapter 4 zooms into the nominal domain and discusses mismatches in grammatical gender under nominal ellipsis in argument and predicate positions. I present cross-linguistically recurrent patterns of well-formed and ill-formed mismatches and argue that the proposed identity condition (coupled with the independently motivated mechanism of repair-by-ellipsis of morphophonological gaps) is necessary and sufficient to account for the attested patterns. I also argue that certain configurations satisfy the identity condition but are ill-formed for other reasons; in particular, ellipsis cannot repair encyclopedic gaps. Extensions of the proposal are discussed in chapter 5, including voice mismatches under sluicing in Austronesian languages, Chung’s generalization, and vehicle change phenomena.