Humor is one of the forms of communication that is most likely to fail. Such humor failure is of interest to humor scholars because it highlights the mechanisms in making humor work (or not). Existing work on humor failure is mainly done in linguistics, and focuses on humor failure as a result of a lack of knowledge. This article argues that humor success and failure depend on Knowledge about Humor: widely shared (though not uncontested), group specific or culture-specific cultural rules and conventions regarding the use of humor. Using examples from humor research, my own empirical work, every-day life and current affairs, I show that different groups have different rules and expectations regarding 1. how, 2. when, 3. by whom and with whom...