Book excerpt: The last research frontier in high frequency electronics lies in the so-called terahertz (or submillimeter wave) regime, between the traditional microwave and the infrared domains. Significant scientific and technical challenges within the terahertz (THz) frequency regime have recently motivated an array of new research activities. During the last few years, major research programs have emerged that are focused on advancing the state of the art in THz frequency electronic technology and on investigating novel applications of THz frequency sensing. This book provides a detailed review of the new THz frequency technological developments that are emerging across a wide spectrum of sensing and technology areas.Volume II presents cutting edge results in two primary areas: (1) research that is attempting to establish THz-frequency sensing as a new characterization tool for chemical, biological and semiconductor materials, and (2) theoretical and experimental efforts to define new device concepts within the ?THz gap?.