Actual innocence is a claim that can be made after the defendant has already been found guilty, and perhaps even incarcerated. In the criminal law, a claim of actual innocence is not equivalent to a not guilty verdict: it is an appeal, using new evidence that was not available at the time of the original trial, meant to undermine the court's confidence in the jury's original verdict. In general, this new evidence must have become available after the trial was completed.
There are generally two situations in which actual innocence may be claimed. The first is when the defendant did not commit the crime for which he was convicted, nor was present, nor participated in any way in that crime. If it can be shown that the defendant was mistakenly identified by witnesses as the perpetrator of the crime and convicted on that account, then a claim of actual innocence can hold. The second situation is when the defendant was falsely accused of a crime he did not commit. If the defendant did not engage in any criminal activity and evidence is presented proving that the original accusation was false, the claim of actual innocence can hold.
There are two basic methods of establishing actual innocence, namely bare claim and constitutional error. A bare claim of actual innocence involves the presentation of hard evidence that casts reasonable doubt on the defendant's conviction, such as new DNA testing on physical evidence. This evidence cannot have been available to the court during the time of the trial. A claim of actual innocence can be made according to constitutional error if gross mistakes were made during the trial that prevented the presentation of evidence that could have proved the defendant's innocence. Examples of this might be suppression of evidence by the court, or the defense attorney's failure to discover a legitimate alibi that existed and present it at trial.