Successful Cadmium Mercury Telluride (CMT) thermal imaging systems have been in production for some time now, but critical materials problems are still present which need to be overcome in order to facilitate development of the next generation of detectors. Currently, there is much interest and effort in the use of mercury and impurity diffusion for diode fabrication. The importance of the understanding and characterisation of diffusion processes to the development of devices and the problems associated with gaining the required fundamental and applied knowledge is discussed. Our current state of knowledge of impurity and self-diffusion is reviewed. It is concluded that a limited amount of diffusion data, useful for identification of possible stable dopants and conversion through mercury annealing, is available for bulk material. There is an urgent need for similar data in epitaxial material along with parallel electrical measurements.