Terahertz spectroscopy can be used to detect explosives and related compounds by the unique absorption lines in the far infrared region. We will present results in both transmission and reflection geometries using a conventional timedomain terahertz spectroscopy system. We illustrate the importance of a uniform reference, and uniform sample preparation. Some problems can be avoided by generalizing this technique to detection with narrow linewidth terahertz illumination and a square law (intensity) detector.