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 Alabama, the rule of comparative fault, also known as comparative negligence, is applied under the doctrine of contributory negligence. Alabama is one of the few states that still follows the traditional contributory negligence rule, which is more stringent than comparative fault. Under this doctrine, if a plaintiff is found to be even 1% at fault for the accident, they are barred from recovering any damages from the defendant. This means that in Alabama, if the plaintiff contributed to their own injury in any way, no matter how minor, they cannot receive compensation from any other party who may have also been negligent. This rule applies even if the defendant is found to be 99% at fault. Alabama's approach is in contrast to the pure comparative fault system, where a plaintiff's recovery is reduced by their percentage of fault, and the modified comparative fault system, where recovery is barred only if the plaintiff's fault reaches a certain threshold (e.g., 50% or more). Alabama's strict contributory negligence standard can have a significant impact on personal injury cases within the state.