The main difficulties of converting the original textual form of medical guidelines to a computer-tractable form are connected both with the ambiguity of the natural language text and with the complexity of the resulting formal (and operational) representation. Proceeding directly from one to the other is thus an extremely demanding task. The proposed Guide-X methodology addresses this problem by breaking the whole process of guideline operationalisation into several steps, each of which requires a different mixture of types (medical, knowledge representation, typographical) and degrees of expertise. The principal technology used is that of XML tagging (using both pre-existing and newly developed languages). The result of each step is connected, element-by-element, to the results of previous steps, thus making the verification and revision of the operationalisation process easier. The methodology is currently being tested in the field of hypertension treatment, within the framework of the Medical Guideline Technology project of the EU Fourth Framework Programme.