Partial differential equations describe processes in which temporal as well as spatial derivatives of the state variables are included in the description of the dynamics. Equations of this kind arise in the modeling of many physical, chemical or biological processes. They are also used in the analysis of communication networks, image processing and logistics. Famous examples are the Maxwell equations of electrodynamics as well as wave and heat equations describing various propagation processes. Partial differential equations are an example of infinite-dimensional systems. We are interested in the dynamics of such systems under the influence of external disturbances and/or control inputs.