A self-consistent model of the L to H transition is derived from coupled nonlinear envelope equations for the fluctuation level, radial electric field shear Er', and pressure gradient Del Pi. These equations exhibit a bifurcation between dual L-mode and H-mode fixed points. They show that the L to H transition has similar characteristics to a phase transition of the second kind for which the order parameter is Er'. The transition occurs when the turbulence drive is large enough to overcome the damping of the E*B flow. This leads to a power threshold condition for the transition Pcrit approximately=( mu nT)edgeaRLs.