When a court makes a ruling in a case, and the appellate court upholds the ruling, or the appellant fails to challenge the ruling, the ruling is considered final and becomes the law of the case. For purposes of that case, the ruling is not subject to reconsideration.
Law of the case is not a doctrine of precedent, or stare decisis, the legal doctrine that a ruling becomes binding in later cases. Rather, it is a matter of procedural law designed to ensure judicial economy. It keeps the courts from becoming bogged down by the appellant raising new matters in subsequent appeals.
The concept of the law of the case, though implicitly in practice throughout Great Britain and the United States for centuries, does not have a basis in statutory law. That is to say, there are generally no statutes or laws declaring that a judge must abide by it. It is an example of common law, court made law, that is, nonetheless, binding on all parties.
The law of the case should not be confused with the concept of case law — the overall body of law established by court rulings. Law of the case concerns the permissibility of a party challenging prior rulings in a single case. So long as the facts do not change, the ruling is considered settled for the purposes of subsequent appeals or if the case is remanded, or sent back, to the trial court.
If the facts of a case do change or if the trial court was clearly wrong, the law of the case no longer applies. For example, if on appeal, an appellate court allowed the introduction of new evidence that shed new light on the facts, the appellate justices would be obligated to reverse a previous ruling. Generally, appellate courts confine their review to findings of law, giving deference to the trial court's findings of facts.
Courts will not, however, apply the doctrine of the law of the case when doing so would create an injustice. They will also reconsider matters that would normally be considered settled if there has been a change in the law — a matter of law relied upon by the trial court has been subsequently overturned or there has been a statutory change that affects the current case.