Abstract. Uniqueness and existence of L ∞ solutions to initial boundary value problems for scalar conservation laws, with continuous flux functions, is derived by L 1 contraction of Young measure solutions. The classical Kruzkov entropies, extended in Bardos, LeRoux and Nedelec's sense to boundary value problems, are sufficient for the contraction. The uniqueness proof uses the essence of Kruzkov's idea with his symmetric entropy and entropy flux functions, but the usual doubling of variables technique is replaced by the simpler fact that mollified measure solutions are in fact smooth solutions. The mollified measures turn out to have not only weak but also strong boundary entropy flux traces. Another advantage with the Young measure analysis is that the usual assumption of Lipschitz continuous flux functions can be relaxed to continuous fluxes, with little additional work.1. Background to Scalar Conservation Laws with Boundary Conditions. DiPerna [11] showed that measure valued solutions are useful to prove convergence of approximations to scalar conservation laws: convergence follows by verifying that the approximations are uniformly bounded in L ∞ , weakly consistent with all entropy inequalities and consistent with the inititial data, cf. also The present work, with continuous flux functions shows that the Kruzkov entropies, in Bardos, LeRoux and Nedelec's sense, are sufficient for L 1 contraction of Young measure solutions, which in turn implies uniqueness of L ∞ solutions. The uniqueness proof uses the essence of Kruzkov's idea with his symmetric entropy and entropy flux functions, but the usual doubling of variables technique is replaced by the simpler fact that mollified measure solutions are in fact smooth solutions. The mollified measures turn out to also have strong boundary entropy flux traces. Existence and uniqueness for the pure initial value problem, with continuous flux functions, was established by semi group methods in [9] and by measure solutions in [20].At the hart of the matter of initial boundary value problems to scalar conservation laws is the trace of entropy fluxes, which define the boundary condition. The first study [2] used solutions with bounded variation and hence their trace exist directly. The work [18] used the equation in the interior domain to show that the entropy flux, *