Ever since the first report of the triboelectric nanogenerator (TENG) in January 2012, its output area power density has reached 500 W m⁻², and an instantaneous conversion efficiency of ∼70% and a total energy conversion efficiency of up to 85% have been demonstrated. We provide a comprehensive review of the four modes, their theoretical modelling, and the applications of TENGs for harvesting energy from human motion, walking, vibration, mechanical triggering, rotating tire, wind, flowing water and more as well as self-powered sensors.