The fluctuation dissipation theorem is at the heart of equilibrium statistical mechanics. For a system away from thermal equilibrium, generalized nonequilibrium fluctuation relations have been found in recent years. Fluctuation relations reveal the fundamental connection between the response of a physical system to a weak externally applied force and the fluctuations in the system without the external force. This 'focus on' series summarizes the present state of the art on nonequilibrium fluctuation relations from classical to quantum statistical physics.