There is an institutional preference for a trial court's rulings and findings in the U. S. judicial system. Thus, for an appellate court to hear an appeal from a lower court the aggrieved party must demonstrate to the appellate court that an error was made at the trial level. The error must have been substantial. "Harmless errors," or those unlikely to make a substantial impact on the result at trial, are not grounds for reversing the judgment of a lower court. Any error, defect, irregularity, or variance, which does not affect substantial rights, shall be disregarded.

Assuming that there was no harmless error, there are two basic grounds for appeal:

1. the lower court made a serious error of law (plain error),
2. the weight of the evidence does not support the verdict.

Plain error is an error or defect that affects the defendant's substantial rights, even though the parties did not bring this error or defect to the judge's attention during trial. Of course, some plain errors or defects affecting substantial rights may be noticed although they were not brought to the attention of the court. In any event, plain error will form a basis for an appeal of a criminal conviction.

It is much more difficult to prevail in an appeal based on the alleged insufficient weight of evidence. Although appellate courts review the transcripts of trials, they almost never hear actual testimony of witnesses, view the presentation of evidence, or hear the parties' opening and closing arguments. Consequently, they are not in the best position to assess the weight of evidence in many cases. For this reason they place much confidence in trial courts' decisions on issues of facts. In an appeal based on an alleged insufficient weight of evidence to support a verdict, the error or misjudgment of evidence must truly be egregious for a defendant to expect to prevail on appeal.