The focus of this thesis is the design and analysis of corrugated conical horn antennas for use in the THz region of the electromagnetic spectrum, for the company SWISSto12®. These antennas are designed to work across the industry standard WR frequency bands, as free-space radiators or as part of a larger quasi-optical network. The main analysis tools used in this work were the in-house mode-matching code, SCATTER, and the commercially available CST: Microwave Studio, which makes use of the finite integration technique. A version of SCATTER which incorporated creation of a corrugated horn antenna’s geometry from user chosen inputs was used to perform parametric sweep analysis when designing the horn antennas. The main design stages were (1) a rectangular to circular transition from a WR rectangular waveguide to a circular waveguide input, (2) a circular smooth-walled to corrugated transition and (3) a transition from a small diameter corrugated waveguide to either a large diameter corrugated waveguide or to free-space. Several styles were examined for the various design stages; the choice of style for the final designs was influenced not only by their performance but also by manufacturing feasibility and cost. Overall, nine individual designs for the WR-1.0. WR-1.5, WR-3.4, WR-5.1, WR-6.5, WR-6.5+, WR-10 and WR-10+ bands have been completed to meet the criteria set by SWISSto12; two of these designs have been manufactured and tested and the results from the measurements performed are presented here.