Negligence.

Amongst unintentional torts one finds negligence as being the most common source of common law. Most Americans are under the impression that most people can sue for any type of negligence, but it is untrue in most US jurisdictions (partly because negligence is one of the few torts for which ordinary people can and do obtain liability insurance.) It is a form of extracontractual liability that is based upon a failure to comply with the duty of care of a reasonable person, which failure is the actual cause and proximate cause of damages. That is, but for the tortfeasor's act or omission, the damages to the plaintiff would not have been incurred, and the damages were a reasonably foreseeable consequence of the tortious conduct.

Some jurisdictions recognize one or more designations less than actual intentional wrongdoing, but more egregious than mere negligence, such as "wanton", "reckless" or "despicable" conduct. A finding in those states that a defendant's conduct was "wanton," "reckless" or "despicable", rather than merely negligent, can be significant because certain defenses, such as contributory negligence, are often unavailable when such conduct is the cause of the damages.

Breach.

Breach is ordinarily established by showing that the defendant failed to exercise reasonable care. Some courts use the terms ordinary care or prudent care instead. Conduct is typically considered to be unreasonable when the disadvantages outweigh the advantages. Judge Learned Hand famously reduced this to algebraic form in United States v Carroll Towing Company:

This means that if the burden of exercising more care is less than the probability of damage or harm multiplied by the severity of the expected loss, and a person fails to undertake the burden, he is not exercising reasonable care and is thus breaching his duty to do so (assuming he has one). In other words, the burden of prevention is less than the probability that the injury will occur multiplied by the gravity of the harm/injury. Under this formula, duty changes as circumstances change—if the cost of prevention increases, then the duty to prevent decreases; if the likelihood of damage or the severity of the potential damage increases, then duty to prevent increases.