This thesis investigates the operation of pulse-excited and continuous-wave terahertz (THz) antennas and develops a model for estimating the power radiated by pulse-excited THz antennas. The simulation tool CST Microwave Studio is used to calculate the input impedance of a number of different antenna types, and a mathematical procedure is developed that gives the power radiated in the time domain as weil as the frequency domain. The predictions ofthe mathematical model are found to be in agreement with values reported from experimental measurements. Furthermore, the effect ofthe length of the dipole antenna on the spectrum of the radiated THz signal in pulse-excited systems is studied. In particular, the origin of the sharp dips that occur in the spectra of these antennas is investigated and explained on the basis ofthe reflection ofthe current pulse that takes place at the ends ofthese antennas. The operation of arrays of THz antennas of different geometrical configurations is also studied and the effect of coupling between the elements ofthe arrays is presented in detail. A new design for continuous-wave THz antennas is proposed that offers higher input impedance to the driving source compared to conventional antennas, and therefore radiates higher power.