In the last few years, pervasive wireless signals have been established as a powerful medium for ubiquitous sensing. Wireless sensing quickly becomes an active cross-disciplinary research area that involves wireless communication, signal processing, human-computer interaction, machine learning, and even hardware design. This paper provides an accessible introduction to this exciting area and presents the key technologies enabling it. It highlights the latest achievements in this field, and the unaddressed research challenges as well as how emerging wireless standards can better support this promising technique.