The massive deployment of Wireless Sensor Networks (WSNs) is expected to increase exponentially in the next few years, allowing millions of wireless devices to work autonomously for new applications. These next-generation WSNs are also expected to interact with other devices such as RFID tags, home appliances, cars, and mobile equipment. When integrated with the cloud, these networks could provide pervasive and ubiquitous services to the users by providing powerful and unlimited storage infrastructure. The current challenges for next-generation WSNs are scala- bility, decentralization, resource scarcity, heterogeneity, and dynamicity. These requirements and challenges cannot be fulfilled by traditional WSNs. In addition, the network, Medium Access Control (MAC), and Physical (PHY) layer protocols developed for traditional WSNs are not applicable to the next-generation WSNs, where millions of battery- powered nodes should operate for longer durations. In other words, the next-generation WSNs require the development of novel protocols at each layer that must be able to extend the network lifetime from months to years. These protocols may allow seamless integration of next-generation WSNs with other networks and platforms including Internet of things and cloud computing.