Evidentials are expressions used to indicate the source of evidence and strength of speaker commitment to information conveyed. They include sentence adverbials such as 'obviously', parenthetical constructions such as 'I think', and hearsay expressions such as 'allegedly'. This thesis argues against the speech-act and Gricean accounts of evidentials and defends a Relevance-theoretic account Chapter 1 surveys general linguistic work on evidentials, with particular reference to their semantic and pragmatic status, and raises the following issues: for linguistically encoded evidentials, are they truth-conditional or non-truth-conditional, and do they contribute to explicit or implicit communication. For pragmatically inferred evidentials, is there a pragmatic framework in which they can be adequately accounted for? Chapters 2-4 survey the three main semantic/pragmatic frameworks for the study of evidentials. Chapter 2 argues that speech-act theory fails to give an adequate account of pragmatic inference processes. Chapter 3 argues that while Grice's theory of meaning and communication addresses all the central issues raised in the first chapter, evidentials fall outside Grice's basic categories of meaning and communication. Chapter 4 outlines the assumptions of Relevance Theory that bear on the study of evidentials. I sketch an account of pragmatically inferred evidentials, and introduce three central distinctions: between explicit and implicit communication, truth-conditional and non-truth-conditional meaning, and conceptual and procedural meaning. These distinctions are applied to a variety of linguistically encoded evidentials in chapters 5-7. Chapter 5 deals with sentence adverbials, chapter 6 focuses on parenthetical constructions, and chapter 7 looks at hearsay particles. My main concern is with how these expressions pattern with respect to the three distinctions developed in chapter 4. 1 show that although all three types of expression contribute to explicit rather than implicit communication, they exhibit important differences with respect to both the truth conditional/ non-truth-conditional and the conceptual/procedural distinctions. Chapter 8 is a brief conclusion.