Research in scalable routing for Mobile Ad Hoc Networks (MANETs) focuses on developing adaptive, efficient, and robust protocols that maintain high performance as the network size and node density increase. Recent studies emphasize hierarchical, cluster-based, and zone-based routing architectures that reduce control overhead and improve route maintenance efficiency. Machine learning and reinforcement learning techniques are increasingly applied to predict link stability, optimize route discovery, and balance network load. Hybrid routing protocols combining proactive and reactive features are also explored to enhance scalability under varying mobility conditions. These innovations collectively ensure efficient resource utilization, reduced latency, and reliable communication in large-scale and dynamic MANET environments.