The article presents data in support of the high prognostic validity of intelligence tests and of their capacity to predict achievement in learning, professional work, and social status. The structural-and-dynamic theory of intelligence developed by the author both offers an explanation of the prognostic power of intelligence tests and draws the line beyond which it may vanish. The author argues that intelligence tests are more than mere measuring tools: using them, psychologists affect their subject. He suggests ways along which the praxiology of testing, that is, a science of the practical use of tests, could develop.