Terahertz radiation, the part of the electromagnetic spectrum between microwaves and infrared radiation, has unique properties in that it easily penetrates through most polymeric materials and is therefore an exciting new tool to study such materials. As well as being a non-destructive imaging probe, in organic molecular crystals terahertz radiation interacts with vibrational modes that extend across large domains of a crystal lattice. This makes terahertz spectroscopy unique: even though it is possible to excite molecules using a variety of energies it is only through the careful selection of the low energy in the terahertz range that it is possible to selectively excite crystal lattice vibrations and study the presence and nature of interactions between molecules.