In this first part of the book, we will introduce the concept of human-in-the-loop cyber-physical systems (HiTLCPSs), through an overview of the evolution of technology and research that has led us to this new paradigm. We will begin with the sensing of “things” and “places”, converging on the more recent phenomenon of sensing of humans and their lives through smartphones in a social networking context. Afterwards, we will try to provide the reader with an overview of the possible types of HiTLCPSs and human roles in them, through a taxonomic analysis. At the end of this part, the reader will have a sense of the extent and challenges of bringing technology closer to understanding humans.