The article provides a comprehensive review of the structure and dynamics of multilayer networks, which are networks where interactions between components can occur through multiple channels or layers. The authors discuss the importance of considering the multiplex nature of real-world systems, which has only recently gained attention due to advancements in data resolution. They cover various aspects, including the redefinition of basic structural measures, the impact of the multilayer nature on processes and dynamics, and the development of generative models for such networks. The article also explores topics such as resilience, percolation, spreading processes, synchronization, and applications in social, technical, economic, climatological, ecological, and biomedical systems. The authors emphasize the need for a generalization of traditional network theory to accommodate the complexity of multilayer networks, highlighting their potential to provide new insights into various scientific fields.The article provides a comprehensive review of the structure and dynamics of multilayer networks, which are networks where interactions between components can occur through multiple channels or layers. The authors discuss the importance of considering the multiplex nature of real-world systems, which has only recently gained attention due to advancements in data resolution. They cover various aspects, including the redefinition of basic structural measures, the impact of the multilayer nature on processes and dynamics, and the development of generative models for such networks. The article also explores topics such as resilience, percolation, spreading processes, synchronization, and applications in social, technical, economic, climatological, ecological, and biomedical systems. The authors emphasize the need for a generalization of traditional network theory to accommodate the complexity of multilayer networks, highlighting their potential to provide new insights into various scientific fields.