Comparative fault—also known as comparative negligence—is a rule of tort law for allocating liability when two or more parties to an accident or liability incident are at least somewhat at fault.

For example, in a case in which both the plaintiff and the defendant were negligent, the jury may be asked to apportion the liability or responsibility for the accident (and the resulting damages) between the parties—usually as a percentage of each party's negligence—and the plaintiff’s recovery against the defendant may be offset or reduced by the amount of the plaintiff’s negligence. This rule may be referred to as pure comparative fault.

Other states have enacted a modified comparative fault statute or law that reduces a plaintiff’s recovery by the plaintiff’s percentage of fault, but bars a plaintiff from any recovery if the plaintiff is 50% or more at fault for the cause of the accident.

And in South Dakota, the comparative fault system uses a slight/gross negligence system and only analyzes the comparative fault if the plaintiff’s negligence is slight and the defendant’s negligence is gross. Under this slight/gross negligence system, if the plaintiff’s negligence is more than slight and the defendant’s negligence is less than gross, the plaintiff is barred or prohibited from any recovery.

Comparative fault laws vary from state to state and may change or evolve at any time—whether they are located in court opinions or in statutes.

In New York, the rule of comparative fault, also known as comparative negligence, is applied in cases where both the plaintiff and the defendant share some degree of fault for an accident or injury. New York follows a 'pure comparative fault' system, which allows a plaintiff to recover damages even if they are 99% at fault, although their recovery will be reduced by their percentage of fault. For instance, if a plaintiff is found to be 30% responsible for an accident and the defendant 70%, the plaintiff's damages award will be reduced by 30%. This system is codified in New York Civil Practice Law and Rules (CPLR) Article 14-A. Unlike some states that have a 'modified comparative fault' rule, which bars recovery if the plaintiff is 50% or more at fault, New York allows plaintiffs to recover damages regardless of their level of fault, as long as there is some fault attributable to the defendant.