In this paper, we study the robust consensus problem for a group of linear discrete-time or continuous-time agents to coordinate over an uncertain communication network, which is to achieve consensus against transmission errors and noises. We model the network by communication links subject to deterministic uncertainties, which can be additive perturbations described by either some unknown transfer functions or norm bounded matrices. We show that the robust consensus problem can generally be solved by solving a simultaneous H-infinity control problem for a set of low-dimensional subsystems. We also derive necessary conditions for the existence of a protocol achieving robust consensus. The results show that for discrete-time agents the uncertainty size must not exceed the inverse of the Mahler measure of the agents, while for continuous-time agents it must be less than the unity. Sufficient conditions in terms of linear matrix inequalities are further presented to design the robust consensus protocols.