理论（英语：Theory），又称学说或学说理论，指人类对自然、社会现象，按照已有的实证知识、经验、事实、法则、认知以及经过验证的假说，经由一般化与演绎推理等等的方法，进行合乎逻辑的推论性总结。

接近科学的学说是科学的，反之则是违背科学的或者说伪科学；任何自然科学的产生，源自对自然现象观察。 人类借由观察实际存在的现象或逻辑推论，而得到某种学说。任何学说在未经社会实践或科学试验证明以前，只能属于假说。如果假说能借由大量可重现的观察与实验而验证，并为众多科学家认定，这项假说就可被称为科学理论。

Theory is a contemplative and rational type of abstract or generalizing thinking, or the results of such thinking. Depending on the context, the results might for example include generalized explanations of how nature works. The word has its roots in ancient Greek, but in modern use it has taken on several different related meanings. A theory is not the same as a hypothesis. A theory provides an explanatory framework for some observation, and from the assumptions of the explanation follows a number of possible hypotheses that can be tested in order to provide support for, or challenge, the theory.