In this last chapter, we will state and prove a theorem of Lebeau [L4] giving a geometric upper bound for the wave front set of the solution of a semilinear wave equation with Cauchy data conormal along an analytic submanifold of the Cauchy hyperplane t = O. The interest of this result is that it is valid in large time, in particular after the formation of caustics. The method of proof relies on the theory developped in Chapter II and Chapter III. In Section 1, after stating the theorem, we display on an example the main ideas of the demonstration.