The phenomenon of substitution failure is a longstanding focus of discussion for philosophers of language. Substitution failure occurs when a change from one co-referential name to another (e.g. from 'Superman' to 'Clark Kent') affects the truth-value of a sentence. Jennifer Saul has shown that this can occur even in the simplest of sentences. She presents the first full-length treatment of this puzzling feature of language, and explores its implications for the theory of reference and names, and for the methodology of semantics.