等效原理（equivalence principle）尤其是强等效原理，在广义相对论的引力理论中居于一个极重要的地位，它的重要性首先是被爱因斯坦分别在1911年的《关于引力对光传播的影响》及1916年的《广义相对论的基础》中被提出来。

等效原理共有两个不同程度的表述：弱等效原理及强等效原理。

对此原理，爱因斯坦曾如是说：「我为它的存在感到极为惊奇，并且猜想其中必有一把可以更深入了解惯性和引力的钥匙。」

What is now called the "Einstein equivalence principle" states that the weak equivalence principle holds, and that:

Here "local" has a very special meaning: not only must the experiment not look outside the laboratory, but it must also be small compared to variations in the gravitational field, tidal forces, so that the entire laboratory is freely falling. It also implies the absence of interactions with "external" fields other than the gravitational field.