The obligation to act with care may arise out of a relationship established by contract, as in the duty assumed by a common carrier (e.g., a railroad) in preserving goods and passengers from damage or injury. But the law also supposes that all persons in the ordinary course of conduct have a duty to avoid inflicting injuries on others. In all noncontractual situations this duty is to act as a "reasonable, prudent person" would act. Injury that results despite such conduct or from circumstances beyond human control (see, e.g., act of God) is not compensable, although the doctrine of strict liability makes those engaged in certain trades and services liable despite non-negligent conduct.

It is usually the function of a jury to determine whether negligence occurred, and the obligation of the plaintiff to demonstrate the defendant's negligence by a preponderance of the evidence. On the other hand, in cases where due care must have been absent (e.g., where a drink bottled at the defendant's plant contains a dead mouse), the judge may apply the doctrine of res ipsa loquitur [Lat.,=the thing speaks for itself] and rule that there was negligence as a matter of law; this obliges the defendant to demonstrate the absence of negligence. In cases where both parties share responsibility for negligence, the law allows reduced damages based on the doctrine of comparative negligence. Thus, a driver who ignored a red light might not recover fully for an injury caused by another driver who was speeding through the intersection; responsibility might instead be assigned, for instance, as belonging 70% to the speeder and 30% to the ignorer of the traffic signal, whose damages for injury would be limited by subtraction from a full recovery.

Negligence law has been of great importance to consumer groups, who have won huge awards of actual and punitive damages, especially from the manufacturers of various goods. In the 1990s business groups and their congressional allies have pushed for federalization of U.S. negligence law, with statutory limitation of forms of damages, arguing that almost all commerce is now interstate and that the threat of large damage awards has been inhibiting American enterprise. Opponents respond that negligence has historically been one of few legal actions useful to the relatively powerless in American society, and that business has not suffered as it claims.

At common law, the right to recover for negligence belonged to the injured party only; his or her death terminated a lawsuit, and heirs might not recover. Today, all jurisdictions have statutes permitting heirs to bring suit for wrongful death and for injuries to the deceased. Negligence claims are the chief source of modern civil litigation. Most cases arise from vehicular traffic accidents; the widespread adoption of no-fault insurance may, however, reduce the role of negligence law in the future. Besides its civil aspects, negligence may also be an aspect of a criminal prosecution, if it results in manslaughter or if it is a serious breach of a public duty (e.g., carelessness by the engineer of a train). In medical, psychotherapeutic, legal, and other professional relationships, negligence, which is measured against generally accepted knowledge and practice standards, is called malpractice.