Human behavior recognition has been considered as a core technology that can facilitate a variety of applications. However, accurate detection and recognition of human behavior is still a big challenge that attracts a lot of research effort. Recent advances in wireless technology (e.g., Wi-Fi channel state information, i.e., CSI) enable a new behavior recognition paradigm, which is able to recognize behaviors in a device-free and non-intrusive manner. In this article, we first provide an overview of the basics of Wi-Fi CSI-based behavior recognition. Afterward, we classify related applications into three granularities, signals, actions, and activities, and then provide some insights for designing new schemes. Finally, we conclude by discussing the challenges, possible solutions to these challenges, and some open issues involved in CSI-based behavior recognition.