The aim of this article is to reflect on the relationship between two disciplines, second language acquisition (SLA) and modern language teaching. The main topic the two disciplines have in common is the learner's language and all the variables which are connected to it. Nowadays, the results of SLA research need to be studied and applied by the teachers, who should respond to learners' needs, respecting their learning strategies. This central role of the learner is essential for modern didactics, which should take into consideration a large number of variables to answer to the learner's motivations and personality, to his linguistic knowledge and his social background. That is the reason why the study of the learner's profile offers the teacher an important opportunity to organize his daily teaching activity respecting and responding to the learner's needs and to the features of his Interlanguage.