Wireless sensor networks (WSNs) are a special type of ad hoc network that can be used in areas where wiring and power supply are difficult, in areas that are inaccessible (for example, because of environmental pollution or war), and in temporary situations (such as when natural disasters occur or fixed communication networks are blocked). They do not require fixed network support and can be rapidly deployed; they have strong invulnerability and can be widely used in military, industrial, transportation, environmental protection, and other fields. Compared with conventional wireless networks, WSNs have severe node energy constraints, and energy exhaustion is the main determinant of WSN performance and the network operation cycle. Many factors affect the energy consumption of nodes, the most important of which is the routing algorithm. However, sensor networks have unique characteristics, and the routing algorithms used in earlier ad hoc networks cannot be applied in them. Therefore, it is necessary to design energy-saving routing algorithms for WSNs.