Multilayer networks are a type of network model that incorporates multiple kinds of relations between nodes. A multilayer approach enables the modeling of an overlapping set of relations of different kind and thus a more realistic representation of many real world complex systems. Ongoing attempts to model real-world systems as multidimensional networks have yielded valuable insight in the fields of economics, social network analysis, urban and international transport, ecology, psychology, medicine, biology, commerce, climatology, physics, computational neuroscience, operations management, and finance.