Resumo: Constitutive theories of gases based on the formal structure of hyperbolic systems of balance laws will be discussed. Such systems are closely related to the Maxwell's transfer equations of the Kinetic Theory of Gases. However, the theories can also be developed within the framework of Continuum Thermodynamics. Boundary value problems for systems of moments for stationary heat conduction will be given as simple examples, which lead to problems of uniqueness related to the boundary values of higher moments.