This article reviews the recent development of op-timal control-based consensus design with incomplete system knowledge. Adaptive dynamic programming, which is one of the most efficient algorithm, is widely used in solving the optimal consensus problem. Specifically, it will first review the optimal consensus problem. Then, the technical derivations for the solution of optimal consensus problem by adaptive dynamic programming will be outlined. Important theoretical problems will be briefly discussed. It will show that adaptive dynamic programming is promising for practical applications in optimal consensus design with incomplete system knowledge.