The paper presents a theoretical model of the H-mode in tokamaks which is based on the bifurcation of the radial electric field at the plasma edge. The electric field is determined by the balance of the non-ambipolar fluxes of ions and electrons at the edge. It is found that bifurcations of the radial electric field, the particle flux and the convective energy loss occur when the edge gradient reaches a critical value. This is attributed to L-H or H-L transition. The critical conditions are examined and the role of neutral particles as well as the effect of impurities are incorporated in the model. Combining the confinement scaling laws for both the core plasma and the scrape-off layer plasma, the threshold power for the transition is derived. The temporal evolution associated with the transition is studied.