Pervasive Computing has developed a vision where the "computer" is no longer associated with the concept of a single device or a network of devices, but rather the entirety of situative services originating in a digital world, which are perceived through the physical world. It is expected that services with explicit user input and output will be replaced by a computing landscape sensing the physical world via a huge variety of sensors, and controlling it via a plethora of actuators. The nature and appearance of computing devices will change to be hidden in the fabric of everyday life, invisibly networked, and omnipresent. Applications and services will have to be greatly based on the notions of context and knowledge, and will have to cope with highly dynamic environments and changing resources. "Context" refers to any information describing the situation of an entity, like a person, a thing or a place. Interaction with such computing landscapes will presumably be more implicit, at the periphery of human attention, rather than explicit, i.e. at the focus of attention. In this chapter we will address some of the Pervasive Computing research challenges and emerging issues of interaction in Pervasive Computing environments. After computing devices pervade into objects of everyday life, computers will be "invisible", but physical interfaces will be "omnipresent"—hidden in literally "every thing". It will contrast implicit and explicit interaction approaches at the frontiers of pervasive, integrated and thus "hidden" technology. In the outlook, we will give a more systematic prospect of emerging lines of research.