Fifth Amendment:
No person shall be held to answer for a capital, or otherwise infamous crime, unless on a presentment or indictment of a Grand Jury, except in cases arising in the land or naval forces, or in the Militia, when in actual service in time of War or public danger; nor shall any person be subject for the same offence to be twice put in jeopardy of life or limb; nor shall be compelled in any criminal case to be a witness against himself, nor be deprived of life, liberty, or property, without due process of law; nor shall private property be taken for public use, without just compensation.
That a defendant may not be retried following an acquittal is the most fundamental rule in the history of double jeopardy jurisprudence.
 1 [T]he law attaches particular significance to an acquittal. To permit a second trial after an acquittal, however mistaken the acquittal may have been, would present an unacceptably high risk that the Government, with its vastly superior resources, might wear down the defendant so that ‘even though innocent he may be found guilty.’
 2 Although, in other areas of double jeopardy doctrine, consideration is given to the public-safety interest in having a criminal trial proceed to an error-free conclusion, no such balancing of interests is permitted with respect to acquittals, no matter how erroneous,
no matter even if they were egregiously erroneous.
 3 Thus, an acquittal resting on the trial judge's misreading of the elements of an offense precludes further prosecution. 4
The acquittal being final, there is no governmental appeal constitutionally possible from such a judgment. This was firmly established in Kepner v. United States, 5 which arose under a Philippines appeals system in which the appellate court could make an independent review of the record, set aside the trial judge’s decision, and enter a judgment of conviction. 6 Previously, under the Due Process Clause, there was no barrier to state provision for prosecutorial appeals from acquittals. 7 But there are instances in which the trial judge will dismiss the indictment or information without intending to acquit or in circumstances in which retrial would not be barred, and the prosecution, of course, has an interest in seeking on appeal to have errors corrected. Until 1971, however, the law providing for federal appeals was extremely difficult to apply and insulated from review many purportedly erroneous legal rulings, 8 but in that year Congress enacted a new statute permitting appeals in all criminal cases in which indictments are dismissed, except in those cases in which the Double Jeopardy Clause prohibits further prosecution. 9 In part because of the new law, the Court has dealt in recent years with a large number of problems in this area.
Acquittal by Jury
Little or no controversy accompanies the rule that once a jury has acquitted a defendant, government may not, through appeal of the verdict or institution of a new prosecution, place the defendant on trial again. 10 Thus, the Court early held that, when the results of a trial are set aside because the first indictment was invalid or for some reason the trial’s results were voidable, a judgment of acquittal must nevertheless remain undisturbed. 11
Acquittal by the Trial Judge
When a trial judge acquits a defendant, that action concludes the matter to the same extent that acquittal by jury verdict does. 12 There is no possibility of retrial for the same offense. 13 But it may be difficult at times to determine whether the trial judge’s action was in fact an acquittal or whether it was a dismissal or some other action, which the prosecution may be able to appeal or the judge may be able to reconsider. 14 The question is whether the ruling of the judge, whatever its label, actually represents a resolution, correct or not, of some or all of the factual elements of the offense charged.
 15 Thus, an appeal by the government was held barred in a case in which the deadlocked jury had been discharged, and the trial judge had granted the defendant’s motion for a judgment of acquittal under the appropriate federal rule, explicitly based on the judgment that the government had not proved facts constituting the offense. 16 Even if, as happened in Sanabria v. United States, 17 the trial judge erroneously excludes evidence and then acquits on the basis that the remaining evidence is insufficient to convict, the judgment of acquittal produced thereby is final and unreviewable. 18
Some limited exceptions exist with respect to the finality of trial judge acquittal. First, because a primary purpose of the Due Process Clause is the prevention of successive trials and not of prosecution appeals per se, it is apparently the case that, if the trial judge permits the case to go to the jury, which convicts, and the judge thereafter enters a judgment of acquittal, even one founded upon his belief that the evidence does not establish guilt, the prosecution may appeal, because the effect of a reversal would be not a new trial but reinstatement of the jury’s verdict and the judgment thereon. 19 Second, if the trial judge enters or grants a motion of acquittal, even one based on the conclusion that the evidence is insufficient to convict, then the prosecution may appeal if jeopardy had not yet attached in accordance with the federal standard. 20
Trial Court Rulings Terminating Trial Before Verdict
If, after jeopardy attaches, a trial judge grants a motion for mistrial, ordinarily the defendant is subject to retrial; 21 if, after jeopardy attaches, but before a jury conviction occurs, the trial judge acquits, perhaps on the basis that the prosecution has presented insufficient evidence or that the defendant has proved a requisite defense such as insanity
or entrapment, the defendant is not subject to retrial. 22 This is so even where the trial court's ruling on the sufficiency of the evidence is based on an erroneous interpretation of the statute defining the elements of the offense. 23 However, it may be that the trial judge will grant a motion to dismiss that is neither a mistrial nor an acquittal, but is instead a termination of the trial in defendant’s favor based on some decision not relating to his factual guilt or innocence, such as prejudicial preindictment delay. 24 The prosecution may not simply begin a new trial but must seek first to appeal and overturn the dismissal, a course that was not open to federal prosecutors until enactment of the Omnibus Crime Control Act in 1971. 25 That law has resulted in tentative and uncertain rulings with respect to when such dismissals may be appealed and further proceedings directed. In the first place, it is unclear in many instances whether a judge’s ruling is a mistrial, a dismissal, or an acquittal. 26 In the second place, because the Justices have such differing views about the policies underlying the Double Jeopardy Clause, determinations of which dismissals preclude appeals and further proceedings may result from shifting coalitions and from revised perspectives. Thus, the Court first fixed the line between permissible and impermissible appeals at the point at which further proceedings would have had to take place in the trial court if the dismissal were reversed. If the only thing that had to be done was to enter a judgment on a guilty verdict after reversal, appeal was constitutional and permitted under the statute; 27 if further proceedings, such as continuation of the trial or some further factfinding, was necessary, appeal was not permitted. 28 Now, but by a close division of the Court, the determining factor is not whether further proceedings must be had but whether the action of the trial judge, whatever its label, correct or not, resolved some or all of the factual elements of the offense charged in defendant’s favor, whether, that is, the court made some determination related to the defendant’s factual guilt or innocence. 29 Such dismissals relating to guilt or innocence are functional equivalents of acquittals, whereas all other dismissals are functional equivalents of mistrials.