This dissertation investigates the interaction of metaphoric and metonymic modes in language and co-speech gesture. The corpus for this study consists of academic discourse and co-speech gestures produced by four American linguistics professors videotaped during introductory courses. The main goal of this work is to offer insights into how cognitive/semiotic principles structure multimodal messages, and how the material properties of each medium determine the cross- modal distribution of semantic features and pragmatic functions. Equal attention is paid to iconicity and metaphor and indexicality and metonymy in spoken discourse and spontaneous representational gestures. Findings suggest that such gestures tend to present spatial dimensions and hierarchical relations of abstract concepts and structures (grammatical categories, syntactic structures, etc.), thus alleviating some communicative pressure that arises when speakers describe highly abstract phenomena. Moreover, the gesture modality reveals dynamic aspects of basic embodied image and sensory-motor schemata that are not necessarily expressed linguistically, yet motivate, at least in part, our understanding of concrete and abstract entities and actions. In chapter 1, I present the main tenets of Peirce's semiotic theory and Jakobson's theory of metaphor and metonymy and suggest ways the semiotic modes these put forward may be applied to co-speech gesture. Chapter 2 introduces my methodology, including the schemes used for discourse transcription and gesture annotation. In chapter 3, I discuss the ways in which Peircean semiotics and cognitive metaphor theory can cross-fertilize each other in relation to iconicity and metaphor in language and gestural modalities (signed languages and spontaneous gesture). In chapter 4, I examine the interplay between icons and indices in gesture, illustrating the different types of iconicity with my data, and proposing several modes of indexicality in gesture. In chapter 5, I first present and illustrate the morphological inventory of the gestures in the data---including salient hand shapes and motion patterns---and then describe the principal metaphorical understandings of basic linguistic units, sentence structure, and linguistic theories, as they surface from the data. The final sections offer insights into the interaction between metaphoric and metonymic modes in gesture, as well as additional metonymic modes from the perspective of contemporary theories of metonymy. (LLBA, Accession Number 200703162, (c) CSA [2007]. All rights reserved.)