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Structured electromagnetic waves carrying orbital angular momentum (OAM) have been explored in various frequency regimes to enhance the data capacity of communication systems by multiplexing multiple co-propagating orthogonal OAM beams (ie mode-division multiplexing (MDM)). Terahertz (THz) communications in free space have gained interest as THz waves tend to have:(a) larger bandwidth and lower beam divergence than millimeter-waves, and (b) lower interaction with matter conditions than optical waves. In this paper, we review recent experimental demonstrations of OAM-based THz MDM communication systems, including (a) THz MDM system with two multiplexed OAM beams;(b) THz OAM multiplexing together with frequency-division-multiplexing and polarization-division-multiplexing;(c) multiplexing a full set of two-dimensional Laguerre–Gaussian (