Diagnostic reference levels (DRLs) for X-ray examinations have been introduced in many countries, among others in most European countries, as a consequence of the Directive on Medical Exposures from 1997. The concept seems to be straight forward, but when implementing it into practice a number of problems arise. The results are dependent on how the dose data are measured and assessed. The interpretation of the results must be performed with great care giving due consideration to the fact that X-ray examinations are very complex and hence not very well characterised by a single figure. In this presentation the various parameters influencing the patient dose are discussed and suggestions are given on how these are managed so as to get the best outcome. When these intentions are followed diagnostic reference levels will be a powerful tool for the optimisation process.