An affidavit is a sworn statement. The affiant, or person giving the statement, signs the written document, swearing that the information contained within is true and acknowledging that he may be charged with perjury if it contains information later proved to be false. Witnesses in criminal cases as well as other legal cases usually give their witness statements as affidavits. If the defendant gives a statement, admission or confession, it is also usually in affidavit form.
Perjury is the crime of lying under oath. Providing false information on an affidavit can frequently form the basis of a criminal case of perjury. In affidavit perjury cases, the affidavit forms the chief evidence used by the prosecution.
An affidavit witness statement forms one of the key tools of police investigation of criminal cases. By obtaining and comparing affidavits from different witnesses, police begin to generate a comprehensive picture of the events and people involved in a complex criminal matter. Investigators may choose to reveal portions of the affidavits they have obtained to other witnesses or the media to encourage additional witnesses to make statements or come forward.
Police must obtain search warrants to search a defendant's or witness's residence or place of business. An officer completes a warrant application affidavit stating the reasons why she needs a search warrant in this particular case and what makes her believe that evidence will be present at that location and time. The judge may ask the officer questions regarding the statements contained in the affidavit. The officer may also reference witness affidavits in the application.
Criminal defendants in American courts are entitled to be released on bail or on personal recognizance, except in the most serious cases. At an initial hearing to consider setting bail, the prosecution may rely on affidavits from witnesses as well as the arresting or investigation police officer to demonstrate to the court that the defendant has a risk of flight or is a particularly violent and dangerous individual. The defense may also submit affidavits attesting to the defendant's work, school, family obligations and other positive ties to the community. In most jurisdictions, the judge has authority to hear these affidavits in lieu of in-person testimony at bail hearings.
As a general rule, affidavits cannot be used at trial in lieu of live testimony of a witness. The U.S. Supreme Court has recently further restricted use of affidavits at criminal trial by ruling that laboratory chemical report certifications---such as certifying that a certain item constitutes cocaine---cannot be admitted into evidence unless the lab analyst is present and testifies regarding the information. Affidavits can be used for purposes of impeaching a witness on cross-examination, if that witness testified differently on the stand than he attested in his affidavit.
Some state courts allow submission of affidavits for consideration in sentencing. Federal courts may permit submission of affidavits on sentencing pertaining to the defendant's argument for a more lenient sentence or the prosecution's support of an agreed-upon sentence or alternative sentencing option; however, prosecution arguments for enhanced sentences must be supported by live testimony and subject to cross-examination.