Abstract
This review summarises the present knowledge of experimental information on frictional effects in nuclear reactions and describes the theoretical approaches in detail. The experimental evidence for nuclear friction stems from reactions where large-scale collective motion is involved, that is fission, deep inelastic heavy-ion scattering and giant excitations. The various theories and models employed are based on the concept that friction, damping and viscosity in nuclei are due to an irreversible flow of energy (and/or linear or angular momentum) from collective to intrinsic motion.