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Abstract Einstein’s equivalence principle was initially expressed in terms of the equivalence of a uniform gravity and an accelerated frame. This principle was challenged by some theorists as invalid or more recently as confusing. Some claim that this principle is no longer useful after the formulation of general relativity. On the other hand, Einstein insisted on the fundamental importance of the principle to his general theory of relativity. Einstein also strongly objected the current infinitesimal version of the equivalence principle, which was actually due to Pauli, as a misinterpretation. Norton found the past objections to Einstein’s theory are due to, to say the least, inadequate understanding although he failed in identifying Einstein’s infinitesimal equivalence principle. It is shown that Einstein’s version does exist in his 1916 paper and his book. Einstein’s version is based on Einstein’s physical insight and is beyond the mathematical theorems from which Pauli rephrased his version. The main difference from Pauli’s version is that a local Minkowski space is obtained through an appropriate choice of acceleration. The related local coordinate transformation must have a physical cause and its consequences must be valid in physics. Einstein’s version requires that his principle is applicable to only a physical space where all physical requirements that include measurements of time dilation and space contraction, must be sufficiently satisfied. For example, before Einstein’s calculation of light bending, the geodesic equation was checked with perihelion of Mercury and time dilation was compared with the formula of gravitational red shifts. Analysis of the case of the uniformly rotating disk clarifies Einstein’s claim of general covariance and the need of restriction by the equivalence principle for a space-time coordinate system of a physical space. 1. Introduction It is generally agreed, as pointed out by Einstein [1], Eddington [2], Pauli [3], Weinberg [4], Misner, Thorne & Wheeler [5], Straumann [6], and Yu [7], that Einstein’s equivalence principle is the theoretical foundation of general relativity. Einstein explained the initial form of his equivalence principle in terms of the uniform gravity and acceleration clearly in 1911 [1,8]. After his principle of general relativity, Einstein proposed his equivalence principle for the general case of a four dimensional Riemannian physical space-time in his 1916 paper [8]. Moreover, through out the remainder of his life, Einstein insisted on the fundamental important of the principle to his theory of relativity. However, a surprising fact is, as Einstein saw it, that except perhaps Eddington [2] none of the theorists seem to understand Einstein’s equivalence principle adequately. Often, after explaining the initial form of Einstein’s principle [1,8], the same theorist would follow essentially Pauli’s “infinitesimal” - principle of equivalence, which is now commonly but mistakenly regarded as Einstein’s version of the principle, although Einstein actually had strongly objected this version of misinterpretation [9]. Einstein’s equivalence principle is also challenged by Synge’s [10] now popular identification of “true” gravitational fields with metrical curvature. John Norton [9], in his article of historical account and analysis, ‘What was Einstein’s Principle of Equivalence?’ claimed that he cannot find Einstein’s own version of infinitesimal equivalence principle from Einstein