As research in the Internet of Thing area progresses, and a multitude of proposals exist to solve a variety of problems, the need for a general principled software engineering approach for the systematic development of IoT systems and applications arises. In this paper, by synthesizing form the state of the art in the area, we attempt at framing the key concepts and abstractions that revolve around the design and development of IoT systems and applications, and draft a software engineering methodology centered on these abstractions.