The active role of the learner in the language learning process has been thoroughly acknowledged in the past fifty years. The discussion on the centrality of the learner in the acquisi- tion of a second (L2) or foreign language (FL) is essentially based on the postulate that the charac- teristics of the learner affect the way in which the second or foreign language is acquired and that successful learners appear to have common personal features. Research on the language learner has demonstrated that every person learning a language has his or her own peculiar features and considerable personal baggage which must be taken into account throughout the learning pro- cess. Individual differences in foreign and second language learning can be identified in character- istics such as age, gender and aptitude, and in factors such as motivation, styles and strategies. This paper will first focus on the development of research on the language learner and on the main issues concerning language learner’s aptitude, motivation, and, in particular, on learner strategies. Age, styles and gender will not be considered as research on these characteristics also involves a deep analysis of sociological and psychological issues which are beyond the aim of this paper. The second part will concentrate on language learner strategies involved in language testing situations and on how the use of these strategies affects the test, its quality and, namely, its validity.