The THz is unique among spectral regions because of the relative infancy of its commercial applications. Much of this infancy has been due to the well known difficulties of generating and detecting radiation. However, the enormous number of important applications in each of the other spectral regions has resulted at least as much from their large in-vestment in systems and applications development - an 'X' factor - as from the technological maturity of the spectral region. Examples in the radio region include magnetic resonance imaging (rf + 'X' = shaped magnetic fields, rf pulse sequences, and signal processing) and cruise missiles (rf = 'X' = rocket and guidance system). In the visible, Night Vision (light = 'X' = electron multiplication and fluorescence) serves as an example.
To grow to maturity, the THz needs not only to optimize its technology for native applications (imaging through ob-scuration, chemical sensing, etc.), but to integrate its attributes with other technologies to address a broader range of challenges. In this paper we will discuss the underlying physics of interactions in the THz to see how they lead to both the attractive and limiting features of the spectral region, while at the same time providing hints about how to overcome these limitations by considering 'X'. Specific examples of 'X' will be provided and the authors will welcome comments, suggestions, and ideas from the audience.