Experimental data on the transition from the low to high confinement mode intokamaks is briefly reviewed, concentrating on those cases where thetransition is made by a slow change in external parameters. The first H-modes,which occurred after the sudden application of neutral beam heating, appearedto result from a bifurcation of the edge transport. However, slow transitionsproduced, for example, by ohmic heating do not have the character of abifurcation but appear to result from a slow and reversible change in thecharacteristics of the edge turbulence, which becomes increasinglyintermittent as the high confinement mode is approached. The experimentalresults are interpreted in the light of various models of the transitionprocess and of the type III or transition edge localized modes that accompanyit. The evidence is mainly against the bifurcation hypothesis but nonlinearprocesses are clearly involved. The implications for the next generation oftokamaks intended to reach thermonuclear ignition are discussed