Dyslexia, also called specific learning disorder, is defined in medical classifications as a lasting difficulty in learning and mastering written language. The purpose of this article is to study the diagnostic process, more precisely the tools used, the actors involved and the types of judgements through which this category is assigned in individual cases. It is based on a qualitative study devoted to an examination of the tools and the professionals involved in this task both in France and the UK. Beyond national specificities, the study shows that diagnosis is performed by clearly identified professions, with the aid of tools which allow for the objectification of gaps. In addition, two diagnostic approaches are highlighted. On the one hand, there is the clinical approach, based on a recognised professional’s clinical gaze, which is a fairly long process aimed at determining the best possible treatment for the case in question. On the other hand, there is the procedural judgement, a depersonalised procedure in which psychometric tests prove decisive.