AbstractExplosives or energetic materials are hazardous, expensive and difficult to handle safely. As such there is a need for simulant explosive materials in order to conduct work without these issues being present. This chapter describes characterisation of a set of simulant materials with respect to millimetre wave and submillimetre wave threat detection technologies. The properties examined were the real and imaginary parts of the relative permittivity. The simulants are based on a modular approach to design appropriate chemical and physical properties which mimic explosives. Collectively, these materials are referred to as Hemtex and can be tailored to match various requirements. The subset of Hemtex materials used in this study were designed to reflect the properties of Semtex and the results of the characterisation showed promise for use as Semtex substitutes.