旧量子论是一些比现代量子力学还早期，出现于 1900 年至 1925 年之间的量子理论。虽然并不很完整或一致，这些启发式理论是对于经典力学所做的最初始的量子修正。旧量子论最亮丽辉煌的贡献无疑应属波耳模型。自从夫朗和斐于 1814 年发现了太阳光谱的谱线之后，夜以继日，废寝忘食，经过近百年的努力，物理学家仍旧无法找到一个合理的解释。而波耳的模型居然能以简单的算术公式，准确地计算出氢原子的谱线。这惊人的结果给予了科学家无比的鼓励和振奋，他们的确是朝着正确的方向前进。很多年轻有为的物理学家，都开始研究量子方面的物理。因为，可以得到很多珍贵的结果。

The old quantum theory is a collection of results from the years 1900–1925 which predate modern quantum mechanics. The theory was never complete or self-consistent, but was a set of heuristic prescriptions which are now understood to be the first quantum corrections to classical mechanics. The Bohr model was the focus of study, and Arnold Sommerfeld made a crucial contribution by quantizing the z-component of the angular momentum, which in the old quantum era was called space quantization (Richtungsquantelung). This allowed the orbits of the electron to be ellipses instead of circles, and introduced the concept of quantum degeneracy. The theory would have correctly explained the Zeeman effect, except for the issue of electron spin.