This chapter is concerned with the consensus of a network of agents with general linear or linearized dynamics, whose communication topology contains a directed spanning tree. An observer-type consensus protocol based on the relative outputs of the neighboring agents is adopted. The notion of consensus region is introduced, as a measure of the robustness of the protocol and as a basis for the protocol design. For neutrally stable agents, it is shown that there exists a protocol achieving consensus together with a consensus region that is the entire open right-half plane if and only if each agent is stabilizable and detectable. An algorithm is further presented for constructing such a protocol. For consensus with a prescribed convergence speed, a multistep protocol design procedure is given, which yields an unbounded consensus region and at the same time maintains a favorable decoupling property. Finally, the consensus algorithms are extended to solve the formation control problems.