When evidence of a defendant's guilt is particularly weak, a judge can grant a "judgment of acquittal" (or "judgment notwithstanding the verdict"), which is nearly the same as an acquittal by a jury. A defendant can make a motion for judgment of acquittal in federal court and in some, but not all, states.
(To learn when defendants are entitled to jury trials, see The Right to Trial by Jury.)
A motion for judgment of acquittal rests on the claim that the evidence at trial was insufficient for a conviction. In other words, the defendant argues that no reasonable jury could possibly find guilt beyond a reasonable doubt.
A defendant can ask a judge to acquit on all of the charges or just some of them. For example, consider a defendant charged with arson and burglary. At trial, the prosecution presents almost no evidence of arson. The defense would be well advised in that situation to bring a judgment of acquittal as to the arson charge. If the judge grants the motion, the jury will decide only whether the defendant committed burglary.
The defense can file a motion for judgment of acquittal either during trial or within a specific time after the jury has reached a guilty verdict. For example, in Pennsylvania, a defendant must file a motion for judgment of acquittal within ten days of the verdict.
A defendant can also make a motion for judgment of acquittal after the prosecution has presented its case, or after the close of all the evidence (before closing arguments).
Judges can't grant a motion for judgment of acquittal just because they think that the jury should have decided the case differently. Instead, viewing it in the light most favorable to the prosecution, the evidence must clearly fail to establish guilt.
In ruling on the motion, a judge—just like a jury—can consider only the evidence presented at trial. So, for example, if a defendant is on trial for arson and has a prior arson conviction that the prosecution couldn't (or didn't) present in evidence, the judge can't consider it.
Winning a motion for judgment of acquittal is usually a longshot. Judges don't like to interfere with the jury process, so the evidence must be bare for them to step in.
If a judge does grant a motion for judgment of acquittal based on insufficiency of evidence, the effect is usually dismissal of the charges and release of the defendant (if the defendant is in jail). The prosecution typically cannot bring the case again because of the prohibition against double jeopardy, which prevents a defendant from being tried twice for the same crime. However, if the judge granted the acquittal after the jury reached a guilty verdict, the prosecution can usually appeal.
For information on a related topic, see Motions for New Trial.