The purposes of this book are twofold. First, the various different methods of accessing the THz range discussed here should serve to convince the reader that there have been qualitative and significant improvements over older, more conventional techniques. It should be clear that these improvements enable practical "real-world" applications of THz technology, in a manner which would not have been possible before. The reader should be convinced that this is a realistic goal within the next few years. Second, the demonstrations and feasibility tests described here should serve as compelling evidence of the utility of such devices. Owing to the unique characteristics of THz radiation and its interaction with materials, these devices have substantial advantages over other, competing technologies in a number of different areas.