‘A world of networks’ considers how the world's networks are interconnected. In cyberspace, power grids support the Internet, which hosts the WWW, which in turn enables email services, social networks, information websites, and file-sharing systems. Many activities, including flight control, bank programs, emergency systems, and commercial services also depend on the WWW. A collapse at one level of cyberspace affects other layers. An organism is the outcome of several layered networks: genetic regulatory networks, protein interaction networks, and metabolic networks are nested into one another. Ecosystems are also interacting networks: antagonistic and mutualistic networks determine how species will thrive. In all these cases, networks provide useful maps to disentangle complex and interwoven systems.