For autonomous Tonelli systems on R n , we develop an intrinsic proof of the existence of generalized characteristics using sup-convolutions. This approach, together with convexity estimates for the fundamental solution, leads to new results such as the global propagation of singularities along generalized characteristics. 1 2 PIERMARCO CANNARSA AND WEI CHENG [4] by solving the generalized characteristic inclusioṅMore precisely, if the initial point x 0 belongs to the singular set of u, hereafter denoted by Sing (u), and is not a critical point of u relative to H, i.e.,then it was proved in [4] that there exists a nonconstant singular arc x from x 0 which is a generalized characteristic. The study of the local propagation of singularities along generalized characteristics was later refined in [36] and [17]. For weak KAM solutions, local propagation results were obtained in [18] and the Lasry-Lions regularization procedure was applied in [12] to analyze the critical points of Mather's barrier functions. An interesting interpretation of the above singular curves as part of the flow of fluid particles has been recently proposed in [27] (see also [34] for related results).Returning to our dynamical motivations, in this paper we try to give an intrinsic interpretation of generalized characteristics and study the relevant global properties of such curves. For this purpose, we use the Lax-Oleinik semigroups T ± t (see, e.g.[24]) defined as follows:where u 0 : R n → R is a continuous function and A t (x, y) is the fundamental solution of (1.1). These operators can be also derived from the Moreau-Yosida approximations in convex analysis ([8]) or the Lasry-Lions regularization technique based on sup-and infconvolutions ([28], [35]). By analyzing the maximizers y t , for sufficiently small t > 0, in the sup-convolution giving T + t u 0 (x) we obtain the global propagation of singularities which represents the main result of this paper. For such a result we need the following assumptions.