This chapter explores the concept of design in MALL. Design can take place at multiple levels, starting with the larger learning environment, to the digital artefacts that are used by learners, and the design of tasks for using the artefacts within the given environment. Some of the key models of general design of the learning environment, artefacts and tasks are provided with examples of how they relate directly to MALL. The complexities of designing assessment are also discussed. The chapter concludes with fundamental guidelines to bear in mind when designing MALL and outlines a list of principles for the successful implementation of mobile devices in language teaching and learning contexts.