Vector network analysis is one of the fundamental measurement techniques applied in radio frequency engineering and therefore holds fundamental importance for Terahertz (THz) communications. It serves to determine the transmission and reflection properties of devices, circuits, antennas, setups or linear time-invariant networks in general. Measurands are the frequency-dependent and complex-valued scattering parameters as defined for an n-port network. The scattering parameters are used to derive component properties (e.g. filter transmission, amplification, mixer attenuation), antenna properties (gain, antenna factor) or transmission channel properties. This chapter aims to give a brief introduction to vector network analyser (VNA) measurements.