In recent years, additive manufacturing has experienced rapid growth, due to its inherent capabilities for creating arbitrary 3-D structures, accessibility and associated low manufacturing costs. This paper first reviews the state of the art in 3-D printing for terahertz (THz) applications and identifies the critical features required for such applications. The future potential for this technology is demonstrated experimentally with the first 3-D printed, optically-controlled THz I-Q vector modulator. Here, miniature high-resistivity silicon implants are integrated into metal-pipe rectangular waveguides. The 3-D printed split-block assembly also includes two packaged infrared laser diodes and a heat sink. The measured performance of a proof-of-principle 4-QAM vector modulator that can operate up to 500 GHz is reported. This new hybrid 3-D printing THz technology, which combines semiconductor devices with potentially low cost, high performance passive guided-wave structures represents a paradigm shift and may prove to be an ideal solution for implementing affordable transceivers in future ubiquitous THz applications.