This chapter describes channel sounding, i.e., the measurement of the characteristics of propagation channels. It starts with a basic discussion of the identifiability of channels ¿ generally channels can be only identified uniquely if they are underspread. Time-domain measurements can be performed by impulse sounders or correlative channel sounders (including the swept time domain cross correlator); the chapter also discusses synchronization between transmitter and receiver, as well as averaging to increase the signal-to-noise ratio. Frequency-domain measurements can be performed with vector network analyzers; chirp sounders are also an attractive alternative. The chapter next describes how to perform directionally resolved measurements, i.e., determine directions of arrival. A data model for signals arriving at an antenna array is established; both beamforming strategies, and high-resolution algorithms (especially Capon¿s beamformer or minimum-variance method, and the ESPRIT algorithm are discussed. We also describe different directional sounder concepts, based on real arrays, virtual arrays, or switched arrays. The generalization of these concepts to MIMO or double-directional sounding concludes this chapter.