The feeling of being offended, as a moral emotion, plays a key role in issues such as slurs, the offense principle, ethics of humor, etc. However, no adequate theory of offense has been developed in the literature, and it remains unclear what questions such a theory should answer. This paper attempts to fill the gap by performing two tasks. The first task is to clarify and summarize the questions of offense into two kinds, the descriptive questions (e.g., what features differentiate offense from similar moral states like anger?) and the normative questions (e.g., what are the conditions for taking offense to be apt?). The second task is to answer these questions by developing what I call ‘the violated norm theory of offense’. According to this theory, feeling offended entails that the norm one endorses is judged to be violated by the offender. Appealing to the violated norm enables this theory to answer the descriptive questions (e.g., taking offense differs from anger because of features like not requiring victims and the difficulty of animal offense) and the normative questions of offense (e.g., taking offense is apt only if the violated norm is universalizable).