Updated March 3, 2016
An alibi is evidence showing that a defendant was somewhere other than the scene of a charged crime at the time that the crime occurred. Film and TV characters often pronounce the word "alibi" with a sneer, suggesting that alibi claims are inherently untruthful. In reality, an alibi is a perfectly respectable legal defense.
Here's how it works: Ellis Ware is charged with robbing a convenience store. Ware offers evidence that he was attending a Kiwanis Club meeting at the time of the robbery. Ware's defense amounts to an alibi.
Defendants may offer an alibi defense without giving up their constitutional right to remain silent. Any witness who can place the defendant at a location other than the scene of the charged crime can provide an alibi.
Let's use the example discussed above as an illustration of how this works. Ellis Ware (who is charged with robbery) chooses not to testify at his trial, fearing that if he takes the stand the judge will allow the prosecutor to attack his credibility by offering evidence of his prior felony conviction. Ware's defense attorney supports Ware's alibi defense by calling Miriam as a witness to testify that she was with Ware at the Kiwanis Club meeting at the time of the robbery. To bolster the alibi defense, Ware's attorney shows Miriam a photograph depicting her, Ware, and others, and asks Miriam to identify the photo as one that was taken during the meeting.
Defendants who offer alibi defenses do not take on the burden of proving to a judge or jury that the alibi is accurate. The burden of proving a defendant guilty beyond a reasonable doubt remains with the prosecution. Of course, a judge or jury can consider the credibility of alibi evidence when deciding whether the prosecution has met its burden.
Back to the Ellis Ware scenario. Miriam testifies on behalf of Ware that he was with her at a Kiwanis Club meeting when the robbery took place. Cross examined by the prosecutor, Miriam admits that she didn't mention this alibi to the police officers who initially interviewed her. During closing argument, the prosecutor tells the jury, "If the alibi were true, Miriam would have said so the first time the police talked to her. She wasn't a believable witness, so you must find the defendant guilty." This argument, without anything more, is improper. Whether or not the jury believes Miriam, the burden remains on the prosecutor to prove beyond a reasonable doubt that Ware robbed the store. Nevertheless, the jury's disbelief of Miriam can add to the persuasiveness of the prosecution's case.
In most states and the federal system, if the defense intends to rely on alibi evidence at trial, "discovery" laws require that it let the prosecutor know beforehand. This gives prosecutors time to investigate the alibi and prepare to undermine it.
For example, let's say Rick Blaine is charged with a sexual assault crime. He plans to offer an alibi defense that he was at the Bijou Theater watching the film Casablanca at the time the assault occurred. Blaine has to notify the prosecution of this planned defense and the witnesses who will support it in advance of trial. The prosecution then has time to investigate the alibi. If the investigation turns up evidence that the Bijou theater was showing only Treasure of the Sierra Madre on the date of the sexual assault, the prosecutor might undermine Blaine's alibi by calling the theater's manager to testify as to which film was showing on the night in question.
For more on the alibi defense, including discussion of federal law on the topic, see Criminal Law Defenses: Alibi. For information about the law in your jurisdiction—and how it applies to your situation—consult a knowledgeable attorney.