ABSTRACTThis paper proposes a new definition of the Efficient Markets Hypothesis with respect to information, which is more formal and precise than those of Rubinstein [13], Fama [4], Jensen [6], and Beaver [1], and which fits well as a framework for interpreting the many tests of the Efficient Markets Hypothesis in the literature. Security markets are here considered “efficient with respect to information set ϕ” if and only if revealing ϕ to all agents would change neither equilibrium prices nor portfolios. In addition to other desirable features, this definition has the “subset property”: efficiency with respect to ϕ implies efficiency with respect to any subset of ϕ.