Mens rea refers to the mental state of a defendant during the commission of a crime and is central to determining criminal liability. It helps differentiate between various levels of culpability, such as intent, knowledge, recklessness, and negligence. These mental states form a hierarchy that plays a key role in distinguishing between crimes like specific intent crimes (e.g., first-degree murder), general intent crimes (e.g., battery), and strict liability offenses (e.g., statutory rape).

Key legal cases, such as Regina v. Cunningham, R v. G, and United States v. Jewell, have shaped the understanding of these mental states, while mens rea also impacts criminal defenses, including mistake of fact, intoxication, and insanity. It further influences the grading of crimes, especially in offenses like homicide and theft, where the intent can differentiate between degrees of the crime.

The Model Penal Code (MPC) standardizes mens rea, offering clear categories that enhance legal analysis. Theoretical and practical implications of mens rea affect plea negotiations, sentencing, and defense strategies, making it a cornerstone of criminal law education and practice.