The paper reviews present understanding of H-mode physics by summarising relevant experimentalobservations and discussing possible interpretation. The most important features of the H-mode area minimum threshold edge temperature (threshold input power) required to achieve the H-mode; thebifurcation nature of the H-transition with instantaneous changes at the plasma edge; and theformation of a transport barrier at the plasma edge leading to pedestals in the density andtemperature profiles. Global energy confinement times, are typically 2× to 3× longer in H-than in L-mode plasmas, reaching, for example, almost 1s in 3MA JET X-point discharges. τE is foundto increase linearly with plasma current. Results of the variation of τE with input power aresomewhat contradictory: no power dependence is found in ASDEX and DIII-D, whereas a degradationwith power is indicated in JET and in JFT-2M limiter H-modes. Correspondingly, predictions forfull power (40MW) 6MA X-point discharges in JET range from 0.6s to >1s, depending upon whichscaling is adopted. Two main theoretical models have been proposed to explain the H-mode with itsheat barrier at the plasma edge. Such a barrier is predicted by the stability properties ofballooning modes close to a magnetic separatrix, corresponding to the transition to a second stableregion above a certain threshold power. It could also arise from a critical temperature gradientmodel based on self-consistent stochasticity.