Many systems of interest in science and engineering can be represented as networks: the internet, the power grid, transport networks, metabolic networks, ecological networks and social networks are just a few of the many well studied examples. The structure of these networks has deep implications for the behaviour of the systems they represent (for traffic flow on the internet, for instance, or the spread of a disease over a human social network). Their structure is complex and we need new tools to help make sense of it. Physics, perhaps surprisingly, has proved a rich source of such tools. This talk will introduce some of the fundamental ideas in this growing field. It will also demonstrate how techniques borrowed from quantum and statistical physics are helping us to understand a wide range of networked systems.