The through-wall system, which applies radio technologies to detect objects behind the wall, can find many appealing applications such as public security, life detection, and medical health monitoring. While being studied for years, the recent advances in high-performance handheld computing devices and artificial intelligence have made the through-wall system more practical. In this article, we present a tutorial-like study on the fundamental radio technologies used in the through-wall system, as well as its recent advances. Different from the traditional through-wall radars, this paper mainly focuses on the handy through-wall techniques with low power, narrow bandwidth, lightweight, no contact, and civil use. Advanced through-wall systems and open research issues are also presented.