This paper is concerned with the use of power properties of tests in econometric applications. Power radius and inverse power functions are deﬁned. These functions are designed to yield summary measures of power that facilitate the interpretation of test results in practice. Simple approximations are introduced for the power radius and inverse power functions of Wald, likelihood ration, Lagrange multiplier, and Hausman tests. These approximations readily convey the general qualitative features of the power of a test.