Metaphor involves the perception of similarities or correspondences between unlike entities and processes, so that one can experience, think and communicate about one thing in terms of another – lives as journeys, minds as machines, emotions as external forces, and so on. A consistent thread in the history of the study of metaphor concerns the potential of different metaphor choices to reflect and facilitate different ways of viewing topics or phenomena – a function of metaphor that is itself metaphorically captured by the notion of “framing.” The related phenomenon of metonymy, although less well studied in these terms, also facilitates framing in discourse. In this chapter, we review research on the framing power of metaphor and metonymy, with a particular focus on studies that are relevant to or directly concerned with the use of metaphor in discourse, broadly conceived. We begin with an overview of rhetorical approaches to metaphor as a tool for persuasion and of cognitive approaches to metaphor as a tool for thinking, including both theoretical and empirical studies. We review a variety of studies that have investigated the framing function of metaphor, and, to a lesser extent, metonymy, in authentic language use from a range of sources (e.g. politics, science and education) and using different qualitative and/or quantitative methods. Focusing on metaphor, where the evidence is most robust, we critically examine the relationship between, broadly speaking, cognitive and discourse-based approaches to metaphor. We go on to provide a concrete example of the framing function of metaphor in healthcare discourse, and show how cognitive and discourse perspectives can be usefully combined into a multilevel analytical framework that can, among other things, be used to make recommendations for professional practice and training.