  Networks are convenient mathematical models to represent the structure of
complex systems, from cells to societies. In the past decade, multilayer
network science -- the branch of the field dealing with units interacting in
multiple distinct ways, simultaneously -- was demonstrated to be an effective
modeling and analytical framework for a wide spectrum of empirical systems,
from biopolymer networks (such as interactome and metabolomes) to neuronal
networks (such as connectomes), from social networks to urban and
transportation networks. In this Element, a decade after the publication of one
of the most seminal papers on this topic, we review the most salient features
of multilayer network science, covering both theoretical aspects and direct
applications to real-world coupled/interdependent systems, from the point of
view of multilayer structure, dynamics, and function. We discuss potential
frontiers for this topic and the corresponding challenges in the field for the
future.
