Wireless sensor networks (WSNs) are becoming increasingly popular, which is changing the way people perceive the world largely, as well as the living styles of human beings. To give readers a basic, wide view of WSNs and make them understood more deeply, this chapter introduces their various aspects briefly, including basic concepts, architectures and protocols, etc. Moreover, it discusses their recent developments, challenges and new trends, based on analysis of many meaningful references. Some classic applications are also shown to approve the popularity of Wireless Sensor Networks.