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 Washington State, the rule of comparative fault, also known as comparative negligence, is applied in cases where multiple parties share responsibility for an accident or injury. Washington follows a pure comparative fault system, as outlined in RCW 4.22.005 through RCW 4.22.015. Under this system, a plaintiff's recovery is reduced by their own percentage of fault. However, unlike modified comparative fault states, there is no threshold of fault (such as 50% or more) that bars recovery. This means that even if a plaintiff is found to be 99% at fault, they can still technically recover 1% of their damages from the other at-fault party. Washington's approach allows for a fair and proportionate allocation of damages based on the degree of fault attributed to each party involved in the incident.