The theory of the L-H transition in tokamaks is discussed. A transport bifurcation model is presented, which predicts reasonably well several properties of the L-H transition in the DIII-D tokamak, including the evolution in time of the density and temperature profiles. A bifurcation in the radial transport occurs because of a nonlinear feedback mechanism involving the radial electric field. A spontaneous transition is predicted when certain conditions are met; no trigger mechanism is necessary. The predicted profile evolution in H-mode can include a transition to VH-mode, with its characteristically wider transport barrier at the plasma edge. Trigger mechanisms are discussed, as well as the idea of considering the L-H transition as a phase transition.