mHealth can offer great potential for the self-management of health conditions and facilitating health services. It is therefore imperative that the design of mHealth systems afford optimum efficacy and effectiveness. Involving end users in collaborative decision making is an essential aspect of increasing acceptance of the treatment intervention. Involving users in the design and evaluation of mHealth systems helps to enable a better understanding of the complexity of user needs and how to incorporate this information effectively into the design process. This chapter discusses how Activity Theory can help to provide a theoretical lens for a User Centred Design framework in the design of mHealth systems. A general overview of Activity Theory and User Centred Design are provided, followed by their application in mHealth. Two use cases are provided that demonstrate how Activity Theory has helped provide a broader contextual analysis to a User Centred iterative approach to system design and evaluation.