Research on Cross-Layer Protocols for Wireless Sensor Networks (WSNs) focuses on integrating interactions among different layers of the network stack-such as physical, MAC, network, and transport to enhance overall performance and energy efficiency. Traditional layered architectures often limit coordination between layers, leading to inefficiencies in dynamic WSN environments. Cross-layer designs overcome these challenges by enabling joint optimization of parameters like transmission power, data rate, routing paths, and congestion control. Recent studies propose energy-efficient cross-layer frameworks that combine MAC scheduling with adaptive routing to extend network lifetime and reduce packet loss. QoS-aware cross-layer schemes are also being developed to maintain reliable communication under varying traffic conditions. By sharing contextual information between layers, these protocols achieve better adaptability, throughput, and scalability, making them ideal for modern IoT and real-time sensing applications.