What branch of government reviews both federal and state court decisions?

Answer 1

Appellate courts in the Judicial Branch of the federal government have jurisdiction (power, authority) to review lower court decisions if the case is sent to them on appeal. Judges don't go looking for cases to review.

Generally,* in order for a state case to be removed to federal court, the case must involve a a federal question. A "federal question" is one that involves matters related to federal law, the US Constitution or a US Treaty. State cases appealed to the US Supreme Court additionally must have exhausted all avenues of appeal in the state court system, and the federal question must have been "preserved," meaning it have been raised at trial and at each subsequent level of appeal.

* There are some exceptions, but including them would make the answer to this question unnecessarily complex (some people may think it already is).

Answer 2

Appellate courts in the Judicial Branch of state and federal government may review lower court decisions if the appellate court receives the case on appeal and has jurisdiction over the case. For example, a federal court decision can't be appealed to a state appellate court.

Answer 3

All three branches. The US Supreme Court (judiciary) has the right of review when a case is presented to it on appeal. The President (federal) or governor (state) is entrusted to review it to ensure the rulings are enforced (executive). Congress (federal) or state legislature (state), has the authority of oversight (legislative). The federal courts can only get involved in a state case when a federal law is at issue or when a US constitutional question is made.