Any interaction refers to the communication between two or more entities (be it abstract/conceptual or physical entity). Successful interaction is equated from the properties of each entity involved in the interaction as well as the capabilities of the interacting entities. With the diversified use and application of computers and specialized devices for specific tasks, such as biomechanical and biomedical devices, interaction design needs to further study the context of the tasks as well. Moreover, with the inclusion of embedded systems and smart devices, instead of focusing only on the hardware performance, the computer architecture needs to consider the opportunities. Especially, HCI can be improved as the current technologies are giving an opportunity for building smart interaction where the user interacts with devices implicitly and in less obtrusive way. In light of this, the design and architecture of an engineered product need to strive for making the product usable and used while making it useful to the user. And this can be achieved if interaction design is dictated by scrutinizing the user model with respects to the usability attributes in view of the context of its task as well as the platform capabilities and constraints as discussed in this chapter.