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 California, the rule of comparative fault, also known as comparative negligence, is applied under a 'pure' comparative fault system. This means that if both the plaintiff and the defendant are found to be at fault for an accident or injury, the court will assign a percentage of fault to each party. The plaintiff's damages award will then be reduced by their own percentage of fault. For instance, if a plaintiff is found to be 30% responsible for an accident and the damages amount to $100,000, the plaintiff's recovery would be reduced by 30%, resulting in a $70,000 award. Unlike modified comparative fault states, where a plaintiff may be barred from recovery if their fault reaches a certain threshold (often 50% or more), California allows a plaintiff to recover damages even if they are found to be 99% at fault, although their recovery would be correspondingly reduced. California's approach allows for some level of recovery regardless of the plaintiff's degree of fault, as long as there is some fault attributable to the defendant.