At common law, a demurrer was the most common pleading by which a defendant would challenge legal sufficiency in criminal or civil cases, but today is the pleading is abolished in many jurisdictions, including the federal court system (though many large jurisdictions, including California retain the demurrer). 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 is not a challenge to the ultimate merits of a case or claim. When ruling on a demurrer, a judge is required by law to assume as true facts alleged in the complaint, even if those facts would later be challenged. Historically, however, a party filing a demurrer often had to admit the facts in the complaint and waive the right to later challenge those facts.
Technically a "demurrer" is not a motion, but a challenge as of right. One does not file a motion for demurrer nor move the court to demur. Rather, a demurrer asks the court to dismiss an action.
In lay terms, if a judge sustains a demurrer, he or she is saying that the law does not recognize a legal claim for the facts stated by the complaining party.
In legal terms, a demurrer attacks or responds to the legal sufficiency of the complaint. The demurring party asserts that the complaint or counterclaim does not amount to a legally valid claim even if the factual allegations contained in the complaint or counterclaim are accepted as true.
Usually, a demurrer attacks a complaint as missing one or more required elements of a claim. For example, a negligence cause of action must allege that: 1) the defendant owed a duty to the plaintiff; 2) the defendant breached the duty; 3) the breach caused plaintiff injury; and 4) the plaintiff suffered damage. A defendant could demur by saying that the complaint failed to plead one or more of these essential elements.
Demurrers may seek to invoke legal principles that protect certain forms of conduct against legal liability, even if that conduct is suspect or morally dubious. For example, if a plaintiff sued a neighbor for negligence, asserting that he or she was owed a duty by the neighbor to assist in protecting against a burglary at plaintiff's home, most all jurisdictions would dismiss this claims because a neighbor has no legal duty to assist protecting against a burglary. Similarly, a complaint for breach of a promise to marry could be met by a demurrer because the law in most jurisdictions expressly prohibits such claims on public policy grounds.
Because a demurrer challeneges legal sufficiency, a judge could reach different outcomes, on the same facts, in different jurisdictions, if the law between the two states differs. In the United States, if a complaining party sued for libel but failed to assert that the allegedly libelous statement is false, the claim could be dismissed on a demurrer or a motion to dismiss. In the United Kingdom, falsity is not a required element of libel and the analogous request to dismiss would not be sustained.
When a cause of action is abolished, a plaintiff who seeks to allege such a cause could be met with a successful demurrer. For example, for many years a spouse who can prove that his or her spouse is sleeping with another could sue the individual sleeping with his or her spouse for alienation of affection. This cause of action having been abolished in most states, a complaint which states such a cause would be dismissed.
Demurrers are decided by the judge rather than the jury. The judge either grants the demurrer by sustaining it, or denies it by overruling the demurrer. In ruling on a demurrer, the judge is required to accept as true all facts written in a complaint.
If a judge sustains a demurrer, he or she may sustain it "with prejudice" or "without prejudice." With prejudice means the plaintiff cannot file another complaint attempting to fix the insufficiencies of the previous complaint. If the demurrer is granted without prejudice, then the plaintiff has leave to correct errors by rewriting the complaint and filing an amended complaint. Demurrers granted with prejudice are rare and reserved for when the judge determines a plaintiff cannot cure or fix the complaint by rewriting or amending it. Because leave to amend is so liberally granted, a demurrer usually is only filed if a plaintiff refuses to repair a fatally deficient complaint or if the complaint cannot ever be supported by law. Many courts will fine or sanction a party who files a demurrer for improper motive, for example, to harass or intimidate the opposing party.
Because a plaintiff can correct errors by amending the complaint, technical or drafting errors are often dealt with by the defendant's lawyer sending a letter to plaintiff counsel. The letter details the errors and typically offers plaintiff counsel the opportunity to file a corrected complaint. Defense counsel do not send the letter to be nice. Most courts require this informal resolution procedure before a party files a demurrer. This is because judges do not want demurrers filed and taking up the court's time when in all likelihood the offending party must be given the opportunity to "repair" the offending complaint.
In order to have an non-meritorious claim dismissed, however, the distinction between the two procedures is that when the Particulars of Claim are struck out, the Claimant usually has another opportunity to file an amended Particulars of Claim, within (say) four weeks, whereas Summary Judgment is final (subject to appeal).
In criminal law demurrer is obsolete, although not formally abolished. It has been superseded by the more modern motion to quash, usually a verbal application to the judge to rule the indictment null and void and to stop the case (demurrer was pleaded in writing).