This paper is devoted to initial boundary value problems for quasilinear symmetric hyperbolic systems in a domain with characteristic boundary. It extends the theory on linear symmetric hyperbolic systems established by Friedrichs to the nonlinear case. The concept on regular characteristics and dissipative boundary conditions are given for quasilinear hyperbolic systems. Under some assumptions, an existence theorem for such initial boundary value problems is obtained. The theorem can also be applied to the Euler system of compressible flow.Keywords symmetric hyperbolic systems, initial boundary value problems, characteristic boundary MSC 35L60, 35L50, 35L40Many problems in mathematical physics can be described by quasilinear symmetric hyperbolic systems. For instance, the system of gas dynamics, the system of shallow water equations and the system of magnetohydrodynamics can be reduced to such systems. In Refs. [1,2] the initial boundary value problems for quasilinear symmetric hyperbolic systems with non-characteristic boundary have been intensively discussed. However, when the boundary is characteristic, only semilinear systems are considered there. For the characteristic boundary case, a related work is given by Ebin [3], who proved the local existence of the boundary value problem for the system of fluid dynamics in a region with rigid wall under the assumptions, that the initial velocity is subsonic, and the initial density is near to a constant. In this paper we will discuss a more general case of the problems for quasilinear symmetric hyper- *