Establishments providing an internal dosimetry service are reliant on models, usually those published by the ICRP, for the interpretation of monitoring data. Historically these were very simple and designed for radiological protection purposes. The use of these models in conjunction with the various published default parameters ensured that doses to workers were most unlikely to be underestimated, but in recent years more realistic assessments have been encouraged. Such assessments are possible where specific data are available, for example information on particle size and solubility. Several examples are presented to demonstrate how the models are implemented and their limitations when applied to actual monitoring data. Based on this experience. key issues relating to the future development of models are discussed and customer needs identified.