A review is presented on the physics of energy release in current sheets during flares. The process of energy storage is discussed and it is shown how the evolution of the magnetic field may give rise to the presence of current sheets. The presently known types of magnetic energy release are discussed and attention is paid to the results of analytical/numerical models and laboratory experiments. Recent results on three-dimensional reconnection are summarized. The observational evidence that flare energy release is a highly time-dependent process puts severe constraints on the models and indicates that nonlinear processes are important. The large scale magnetic structure in which sheets are embedded is discussed for both compact and two-ribbon flares. Apart from the MHD-picture of current sheets some plasma physical aspects are discussed together with the energy partitioning in sheets (acceleration/heating).