This thesis presents the first experimental study of the propagation characteristics and field distribution of surface plasmon polaritons (SPPs) at terahertz (THz) frequencies. A measurement setup has been designed which allows the generation, demonstration and systematic investigation of SPPs at flat surfaces of various materials. In this chapter a general introduction is given which comprises the different concepts that are involved in the presented experiments. After giving a general introduction to electromagnetic interface excitations in section 1.1, a qualitative account of the SPP is found in 1.2. In section 1.3 we discuss the peculiarity of the THz frequency range in which experiments have been carried out, followed by the conceptual formulation of this thesis, including a short summary of the subsequent chapters (section 1.4).