Interconnected networks—systems composed of multiple interacting network layers—have emerged as a critical paradigm for understanding complex systems across diverse domains, from critical infrastructure and biological organisms to social-technical ecosystems and financial markets. Unlike isolated single-layer networks, interconnected networks exhibit unique emergent phenomena including cascading failures, interlayer synchronization, and robustness trade-offs that cannot be predicted from the analysis of individual layers alone. This review synthesizes recent advances in the theory and applications of interconnected networks, examining their structural organization, dynamic behavior, resilience properties, and control strategies. We discuss how interdependencies between network layers can amplify both functionality and fragility, presenting both theoretical frameworks and empirical evidence from infrastructure systems, biological networks, and social platforms. Key findings include the identification of robust-yet-fragile characteristics in interdependent systems, the role of interlayer degree correlations in determining system resilience, and the emergence of novel critical phenomena at coupling boundaries. We conclude by identifying critical gaps in current understanding and proposing directions for future research, including the need for adaptive control mechanisms, real-time monitoring of interlayer dynamics, and the development of unified theoretical frameworks that bridge structural and dynamical perspectives.

Keywords: Interconnected networks, multilayer networks, interdependencies, cascading failures, resilience, synchronization, critical infrastructure.