We obtain and study new $Phi$-entropy inequalities for diffusion semigroups, with Poincare or logarithmic Sobolev inequalities as particular cases. From this study we derive the asymptotic behaviour of a large class of linear Fokker-Plank type equati
ons under simple conditions, widely extending previous results. Nonlinear diffusion equations are also studied by means of these inequalities. The $Gamma_2$ criterion of D. Bakry and M. Emery appears as a main tool in the analysis, in local or integral forms.
