The role of the judiciary is to decide cases and controversies between adversarial parties, including the government. Through the concept of stare decisis, judicial decisions in United States jurisdictions can act as binding precedent for subsequent decisions. In most cases, when an appellate court makes a decision it not only decides who wins the specific case, but also provides a detailed written opinion that explains the basis for the court's decision to guide lower courts in handling future cases.

Every level of the federal courts has the power to interpret the Federal Constitution and federal laws and regulations. The courts also exercise judicial review over federal statutes and agency actions, and determine the constitutionality of federal and state laws. To the extent any statute or agency action is found to be unconstitutional, it is invalid. Federal courts also interpret federal legislation and federal agency rules and decisions.

Judicial Review of Agency Action
Many federal environmental statutes provide specific standards for judicial review of agency actions under the statute. In the absence of any specific statutory review procedures, the grants a general right of judicial review of any adverse, final agency action. The reviewing court can decide all relevant questions of law, interpret constitutional and statutory provisions, and determine the meaning or applicability of the agency action. The reviewing court has the authority to compel any agency action unlawfully withheld or unreasonably delayed, or to set aside any agency action, findings, or conclusions the court finds to be: (A) arbitrary, capricious, an abuse of discretion, or otherwise not in accordance with law; (B) contrary to constitutional right, power, privilege, or immunity; (C) in excess of statutory jurisdiction, authority, or limitations, or short of statutory right; (D) adopted without procedures required by law; (E) unsupported by substantial evidence in administrative cases; or (F) unwarranted by the facts to the extent that the facts can be reviewed by the court. Many judicial challenges to administrative agency rules go directly to a court of appeals and are not further tried by the district courts.

Common Law
The United States is a common law country. The legal system of every state is based on the common law, except Louisiana (which is based on the French civil code). Common law has no statutory basis; judges establish common law through written opinions that are binding on future decisions of lower courts in the same jurisdiction. Broad areas of the law, most notably relating to property, contracts, and torts, are traditionally part of the common law. These areas of the law are mostly within the jurisdiction of the states, and thus state courts are the primary source of common law. The area of federal common law is primarily limited to federal issues that have not been addressed by a statute.

Judicial Procedures
All courts follow a strict set of procedural requirements. In 1938, the Supreme Court promulgated the Federal Rules of Civil Procedure, which are periodically updated and renewed by the United States Judicial Conference. They are uniform in all federal jurisdictions, although each federal court may also adopt additional rules. Every state court has its own set of rules, which are typically not as detailed or strict as the federal rules. In courts of original jurisdiction, judges are usually provided with juries to decide all questions of fact. The right to a jury is generally guaranteed by the federal Constitution in federal cases, and state constitutions typically contain similar provisions which apply in state cases.