Terahertz (THz) frequency range usually refers to the electromagnetic spectrum between 100 GHz and 10 THz, which is between the millimetre and infrared regions. THz research has received a lot of attention because of its wide potential applications for such as high-speed wireless communications, medical imaging, remote sensing and security scanning. Photoconductive antenna is the most popular device used to generate and detect THz waves. However, there are still many challenges in this area, for example, how to improve its radiation efficiency and how to increase its directivity to the desired direction. In this dissertation, firstly four methods are proposed to improve the generation efficiency of photoconductive antennas. The first method is to adjust the gap of the photoconductive antenna to an optimum value which is dependent on the input laser power and the material properties of the substrate. The second method is to focus the laser beam on a very small area rather than the whole gap and the generated power can be increased by more than 5 times. The third method is to increase the bias voltage, which can strengthen the photo-induced current. The final method discussed is to use the indentation configuration instead of the conventional dipole shape to enhance the electric field in the gap which can result in about two times stronger power radiation. Secondly a THz hom structure is introduced to improve the directivity and the radiation efficiency of the photoconductive antenna. The conventional photoconductive antenna cannot provide high directivity, but this horn antenna can if it is designed and constructed properly. It consists of two main parts: a photoconductive emitter and a THz conical horn. A computer aided design approach has been adopted, and the simulation results show that the THz conical horn antenna with the proposed feeding structure can radiate more THz power in desired directions than conventional antenna. The directivity of this structure is proved to be 10 dB greater than the conventional photoconductive antennas. It should be pointed out that the THz horn antennas are not the same as the conventional microwave horn antennas. The major difference is on the feeding structure. In addition, the effects of the substrate on THz photoconductive antennas are also investigated theoretically and numerically, some very interesting results are obtained.