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 North Carolina, the rule of comparative fault is not followed in the traditional sense. Instead, North Carolina adheres to the doctrine of contributory negligence. Under this doctrine, if a plaintiff is found to be even slightly at fault for the accident or injury, they are barred from recovering any damages from the defendant. This is one of the strictest forms of negligence law in the United States, as it allows for no recovery if the plaintiff's own negligence contributed in any way to the harm suffered, regardless of the degree of fault apportioned to the defendant. North Carolina's adherence to contributory negligence is an exception in the modern trend where most states have adopted some form of comparative negligence system.