With the increase of communication frequency, terahertz (THz) communication technology has been an important research field; particularly the terahertz modulator is becoming one of the core devices in THz communication system. The modulation performance of a THz communication system depends on the characterization of THz modulator. THz modulators based on different principles and materials have been studied and developed. However, they are still on the way to practical application due to low modulation speed, narrow bandwidth, and insufficient modulation depth. Therefore, we review the research progress of THz modulator in recent years and evaluate devices critically and comprehensively. We focus on the working principles such as electric, optical, optoelectrical, thermal, magnetic, programmable metamaterials and nonlinear modulation methods for THz wave with semiconductors, metamaterials, and 2D materials (such as graphene, molybdenum disulfide, and tungsten disulfide). Furthermore, we propose a guiding rule to select appropriate materials and modulation methods for specific applications in THz communication.
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