This chapter explores the internal model principle as a tool for robust synchronization in heterogeneous systems. It builds on conventional output regulation techniques, adapting them for networked environments. In such environments, the exosystem, which serves as the reference model, is subject to external perturbations due to network interconnections, resulting in a robust perturbed output regulation problem. The chapter demonstrates that this problem can be translated into an input-to-state stabilization problem through the internal model design. Finally, the input-to-state stabilization problem is solved for lower-triangular systems.