Networks constitute the backbone of complex systems, from the human brain to computer communications, transport infrastructures to online social systems and metabolic reactions to financial markets. Characterising their structure improves our understanding of the physical, biological, economic and social phenomena that shape our world. As 
a result, complex networks have become an essential ingredient in the background of any scientist [1]. In this seminar I will present an overview of the new theory and methods of network science, of the main results found, and of some of the still open challenges. 
I will concentrate, in particular, on the structure and dynamics of multi-layer networks [2][3], namely multiplex networks and temporal networks, discussing cases where the presence of many layers gives rise to the emergence of novel behaviours, otherwise unobserved in single-layer networks [4][5].  


[1] V. Latora, V. Nicosia and G. Russo, Complex Networks: Principles, Methods and Applications, Cambridge University Press (2017) 
[2] Structural measures for multiplex networks Battiston, Nicosia, Latora, Phys. Rev. E89, 032804 (2014) 
[3] Growing multiplex networks V. Nicosia, G. Bianconi, V. Latora, M. Barthelemy, Phys. Rev. Lett. 111, 058701 (2013) 
[4] Collective phenomena emerging from the interactions between dynamical processes in multiplex networks, Nicosia, Skardal, Arenas, Latora, Phys. Rev. Lett. 118, 138302 (2017)
[5] Mobility and congestion in dynamical multilayer networks with finite storage capacity Manfredi, Di Tucci, Latora, Phys. Rev. Lett. 120, 068301 (2018)