在逻辑中，正规模态逻辑是模态公式的集合 L，L 包含

• 所有命题重言式，
• Kripke 模式: ，

并且 L 闭合于

• 代换，
• 分拆规则: 从 A 和 A→B 推出 B，
• 必然性规则: 从 A 推出 。

最小化的正规模态逻辑叫做 K。

In logic, a normal modal logic is a set L of modal formulas such that L contains:

and it is closed under:

The smallest logic satisfying the above conditions is called K. Most modal logics commonly used nowadays (in terms of having philosophical motivations), e.g. C. I. Lewis's S4 and S5, are extensions of K. However a number of deontic and epistemic logics, for example, are non-normal, often because they give up the Kripke schema.