The concept of test validity has gained importance since psychological tests have been used. Validity is related to the degree of measurement capability of a certain test, which intends to measure a desired property. However, from the years which the concept of validity emerged to the present day debates about definition and classification of validity and the interpretation of its results are on-going. In this study all debates about validity from the past to the present day are examined. Originally the classification of validity was the basis of criterion referenced validity, since then, the basic psychological structure has shifted to be associated with construct validity. In this approach all types of validity were unified under construct validity. Unified validity, as this approach is called, indicates that all evidence for validity of test data revealed construct validity. According to opponents of this view, construct validity, which is more a theoretical study, would not be sufficient to describe the actual validity of the tests used in education. Today their are views defending the unified validity approach as well as views against it and debates about the problems of holding onto content validity under the umbrella of construct validity still continue to this day. At the conclusion of all these debates, it seems that there is consensus that validity is an important evidence gathering process.