Discussions of the role of theory with regards to technology in language teaching have been rather limited to date and have typically relied heavily on second-language acquisition (SLA) theories or other theories related to learning and pedagogy. This chapter provides an overview of the nature of theory, exploring concepts such as informal and formal theories, and describes the fundamentals of what theories are and their function. Theories as they are applied to CALL are explained, along with how they relate to theories in MALL. Theories that are pertinent to MALL implementation are outlined in terms of theories of technology, theories of complexity, theories of motivation and social models of language learning. Each is described with examples from the literature which show how they can help to understand what happens while learning languages with mobile devices.