Quality and quantity in test validity: how can we be sure that psychological tests measure what they have to?

During the twentieth century, many authors have offered their contributions to the controversial subject of test validity. Thus, the concept of validity seems to be quite a simple idea until one looks at the literature on the subject. Validity can be defined as “the degree to which the test actually measures what it purports to measure,” but the question of how to define validity and how to test it is both an old question and a never-ending story. We have to deal with it every time we are asked to use a test that already exists or to construct a new one. Therefore, the article aims at dealing with test validity in order to point out the different (qualitative and quantitative) aspects of validity, which have emerged from empirical research and theoretical reflections. It is thought as a critical review which can be useful both to theoretically oriented and practically inclined psychologists.