A multidimensional channel sounder is necessary for the experimental characterization of time-variant multipath propagation in its joint delay, direction, and Doppler domains. In this chapter, we refer to the fundamental principles of sounding architecture design and geometric modeling of multipath wave propagation, which form the basis for model-based parameter estimation from measurements. The basic design features, requirements, and parameters of wideband spread spectrum excitation signals, mutual Tx/Rx synchronization, radio access, and data recording are explained. The specific challenges of the sounder antenna architecture for resolution and spatial filtering of propagation directions and polarization orientation in the THz frequency range are elaborated. This chapter provides an overview of the statistical analysis of spatial communication channels and introduces a parametric multipath model, which forms the basis for model-based system identification.