Friction is a fundamental phenomenon that affects many different areas of human life and technology. In this Colloquium the microscopic origins of friction are reviewed in light of recent developments that are beginning to unveil the processes at the interface of two sliding materials. Large-scale simulations and theoretical modeling help reveal the complex nature of this basic phenomenon which is still the subject of much controversy. The physics of sliding friction is gaining impulse from nanoscale and mesoscale experiments, simulations, and theoretical modeling. This Colloquium reviews some recent developments in modeling and in atomistic simulation of friction, covering open-ended directions, unconventional nanofrictional systems, and unsolved problems.