We study scalable, distributed, and adaptive routing algorithms for communication networks. The back-pressure algorithm introduced in  is a well-known distributed and adaptive routing/scheduling algorithm where nodes only need the queue length information of neighboring nodes to make routing decisions, and packets are adaptively routed in the network according to congestion information, which makes the algorithm resilient to traffic and topology changes. However, the back-pressure algorithm requires routers to maintain a separate queue for each destination, which prevents its implementation in large-scale networks like the Internet. In this paper, we propose a cluster-based back-pressure routing algorithm, which retains the distributability and adaptability of back-pressure routing, while significantly reducing the number of queues that have to be maintained at each node. Since the cluster-based algorithm performs adaptive load-balancing in the network, it has the potential to eliminate the need for off-line traffic engineering in the Internet.