A demurrer is a pleading in a lawsuit that objects to or challenges a pleading filed by an opposing party. The word demur means "to object"; a demurrer is the document that makes the objection. Lawyers informally define a demurrer as a defendant saying "So what?" to the pleading.

Typically, the defendant in a case will demur to the complaint, but it is also possible for the plaintiff to demur to an answer. The demurrer challenges the legal sufficiency of a cause of action in a complaint or of an affirmative defense in an answer. If a cause of action in a complaint does not state a cognizable claim or if it does not state all the required elements, then the challenged cause of action or possibly the entire complaint can be thrown out (informally speaking) at the demurrer stage as not legally sufficient. A demurrer is typically filed near the beginning of a case in response to the plaintiff filing a complaint or the defendant answering the complaint.

In common law, a demurrer was the pleading through which a defendant challenged the legal sufficiency of a complaint in criminal or civil cases. Today, however, the pleading has been discontinued in many jurisdictions, including the United Kingdom, the U.S. federal court system, and most U.S. states (though some states, including California, Pennsylvania, and Virginia, retain it). In criminal cases, a demurrer was considered a common law due process right, to be heard and decided before the defendant was required to plead "not guilty," or make any other pleading in response, without having to admit or deny any of the facts alleged.

A demurrer generally assumes the truth of all material facts alleged in the complaint, and the defendant cannot present evidence to the contrary, even if those facts appear to be obvious fabrications by the plaintiff or are likely to be easily disproved during litigation. That is, the point of the demurrer is to test whether a cause of action or affirmative defense as pleaded is legally insufficient, even if all facts pleaded are assumed to be true.

The sole exception to the no-evidence rule is that a court may take judicial notice of certain things. For example, the court can take judicial notice of commonly known facts not reasonably subject to challenge, such as the Gregorian calendar, or of public records, such as a published legislative report showing the intent of the legislature in enacting a particular statute.