Due to the use of highly directional antennas in terahertz (THz) communication systems, it is often assumed that eavesdropping and attacks are not possible. However, various evaluations based on both measurements and simulations have shown the possibility of eavesdropping even with the use of highly directive antennas. In this chapter, eavesdropping aspects are discussed for three different scenarios: reflections within an indoor environment, reflections at scattering objects, and reflections caused by a moving person. For the analysis, the metrics of secrecy capacity and blockage are introduced.