When Congress removed all statutory barriers to Government appeals in 1971, the U.S. Supreme Court began to look closely at policies underlying the double jeopardy clause in order to clarify restrictions on Government appeal rights. The controlling constitutional principle adopted by the Court has been to prevent multiple prosecutions, as demonstrated in the cases of United States v. Wilson and United States v. Jenkins where the Government appealed post-verdict rulings. Cases involving Government appeals of pretrial orders and midtrial acquittals, notably Serfass v. United States and Fong Foo v. United States, are analyzed, as are appeals of acquittals after a mistrial and appeals of post-verdict acquittals. These decisions demonstrate that the Government is allowed to appeal whenever a second prosecution will not be necessary, and thus any appeal which will result in the reinstatement of a guilty verdict is permitted. Accordingly, the Government can appeal any post-verdict order and any order granted before the attachment of jeopardy. Conversely, appeals which will result in a second prosecution, even if a full trial is not required, are denied. This approach bars any appeal from a midtrial acquittal or an acquittal after a mistrial. Contrary to the principle of preventing multiple prosecutions, lower courts have allowed the Government to retry a defendant when reversal is for a procedural error or insufficient evidence. Legal rationales underlying these decisions are examined, particularly weaknesses in the affirmative waiver theory which has been used since 1950 to permit retrials when the reversal was for insufficient evidence. Courts faced by stare decisis to follow the affirmative theory have criticized the doctrine and have been reluctant to follow it, as seen in the Seventh Circuit Court's 1977 decision in Sumpter v. DeGroote. A fairness rationale which tries to balance society's interest in punishing the guilty against the defendant's right to a fair trial should be applied uniformly to all reversals. A Supreme Court decision on this issue is needed to guide the actions of lower courts. Approximately 130 footnotes are included.