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 Montana, the rule of comparative fault, also known as comparative negligence, is applied under the Montana Code Annotated (MCA) § 27-1-702. This statute adopts a pure comparative fault system. Under this system, when an accident occurs and more than one party is found to be at fault, the court or jury will assign a percentage of fault to each responsible party. A plaintiff's recovery is then reduced by their own percentage of fault. For example, if a plaintiff is found to be 30% at fault for an accident and the damages amount to $100,000, the plaintiff's recovery would be reduced by 30%, allowing them to recover $70,000. Unlike modified comparative fault states, Montana does not bar recovery if the plaintiff is 50% or more at fault. The plaintiff can still recover damages in Montana even if their fault is greater than the defendant's, but their recovery will be accordingly reduced.