This chapter extends the internal model principle to an output communication setting, which involves both the perturbed consensus and perturbed output regulation problems discussed in previous chapters. Moreover, the coupling of these problems requires a small-gain condition to ensure system stability and achieve the ultimate goal of synchronization. Meeting this condition involves solving an input-to-state stabilization problem with a specified gain, for which a complete solution is provided for lower-triangular systems. The chapter also examines synchronization challenges in frequency-modulated multi-agent systems and explores how small-gain techniques can be leveraged to address these challenges.