The traditional view of Internet of Things (IoT) attempts to connect all the physical objects to build a global, infrastructure-based IoT. In this paper, however, we will present opportunistic IoT, which is formed based on the ad hoc, opportunistic networking of devices (e.g., mobile phones and smart vehicles) using short-range radio techniques (e.g., Bluetooth and Wi-Fi). The opportunistic IoT demonstrates inherently the close relationship between human and opportunistic connection of smart things. It enables information forwarding and dissemination within and among the opportunistic communities formed based on the movement and opportunistic contact nature of human. We characterize the bi-directional effects between human and opportunistic IoT, discuss the technical challenges faced by this new research field, and propose a reference architecture for developing opportunistic IoT systems. Some of our ongoing practices, including opportunistic mobile social networking, opportunistic marketing, and community service provision are further presented to demonstrate the potential application areas and technical solutions of opportunistic IoT