Although its theoretical basis may be disputed, nobody questions the proposition that a person charged with a crime has a constitutional right to present a defense. Presenting a defense naturally requires access to proof. Access includes not only the availability of evidence, but also its permissible use. Consider some examples: A defendant wants to testify, but his lawyer's threats drive him off the stand. A witness who might be expected to give favorable testimony for the defense appears at trial but refuses to testify. A defense witness wants to testify, but because the defendant failed to notify the prosecutor about the witness, is precluded from giving such proof. Evidence that might exculpate a defendant has been suppressed by the prosecutor, or has been lost or destroyed by the police. The evidence in all of these cases as a practical matter is inaccessible to support the defense. Does the defendant have any remedy? This article discusses the types of evidence that theoretically are accessible to a defendant, some of the practical and legal barriers that can obstruct a defendant's access to such proof, and the constitutional protections afforded a defendant if access is impermissibly denied.
Bennett L. Gershman, The Right to Evidence, N.Y. St. B.J., Nov. 1989, at 52, http://digitalcommons.pace.edu/lawfaculty/623/.