The British Education Act of 1944 stipulated that instruction and training be offered according to the ages, abilities, and aptitudes of pupils. One specific problem concerned the entry to secondary schools of pupils from a variety of primary schools. The resulting problem of determining the different aptitudes and abilities has been partially solved by the use of standardized tests. This pamphlet is designed to provide a brief introduction to the methods of constructing and using standardized tests, and to discuss special difficulties encountered in the construction and use of standardized tests in Wales, a mixed language area. Specifically discussed are the various kinds of standardized tests, what is meant by standardization, what such tests determine, the choice of tests, comparison of the results of various tests, and the concepts of mental age, attainment ages and quotients. (CLK)