This study falls within the sphere of the didactics of specialized translation, more exactly, the field of French-Spanish legal translation. Its purpose is to make some headway in the analysis and characterization of an aspect which has received little attention to date: the subject knowledge of legal translators. It is true that there is general agreement among scholars that translators require a thorough knowledge of the legal systems between which they translate. It is no less true, however, that the question just how this knowledge is acquired has not received specific attention nor has it been analyzed in detail (is generally approached by means of comments of a general nature, with no attempt at detailed analysis). From the point of view of translator training, what would seem necessary would be a clearer understanding of exactly what type of knowledge our students require about legal systems and how this knowledge can be developed. Undoubtedly, adjectives applied to knowledge, such as 'passive' and 'useful' provide a first step in an approach towards the nature of subject knowledge required for the translation of legal texts, as they highlight the fact that the subject knowledge the translator needs does not have to be at the same level as that of the legal expert, since we are dealing with two different types of knowledge. Starting from this point, this paper revises a series of notions which we consider fundamental for legal translators if they are to have a good command of their subject knowledge. Among these are: comparative law, self-study, strategic competence and the acquisition of documentary resources.