In this paper, the cooperative output regulation (COR) of heterogeneous linear multi-agent systems is studied. We propose a novel distributed observer approach that ensures synchronization of agents’ outputs to a reference trajectory generated by a leader, while rejecting disturbance. A unified framework based on tools from ${\mathcal {H}_{\infty} }$ theory is established which allows treatment of both networks of non-introspective and networks of introspective agents. The proposed protocols do not require exchange of the controller states, which reduces or completely eliminates communication costs. A sufficient local stability condition is derived and a novel controller gain design method is provided to satisfy that condition. It is proven that the solvability of the COR problem can be guaranteed in advance for: i) introspective agents with arbitrary dynamics, ii) non-introspective agents with stable dynamics, under the assumption that the poles of exosystems lie on the imaginary axis. The effectiveness of the proposed approach is verified through numerical simulations.