Unlike rules governing discovery in civil cases, which require that the two sides exchange most information about their respective cases, criminal discovery result in a much more limited flow of information. Many commentators, for many years, have called for the liberalization of criminal discovery statutes and rules. Indeed, some states have heeded the call. But about a dozen states follow the highly restrictive federal rule, which is premised in part on the idea that a defendant should not be entitled to witness names or statements for pretrial investigation, but rather only for cross-examination purposes should the case ever get to that stage. The remaining states fall between the two models. New York is on the restrictive end of the spectrum. Investigation of the prosecution’s case and possible defenses has long been recognized as a core function of defense counsel in a criminal case, one that is necessary to the testing of the facts in our adversarial system. This function has been constitutionalized as the Sixth Amendment duty to "make reasonable investigations or to make a reasonable decision that makes particular investigations unnecessary.” This article describes the development of that duty, especially as it relates to recent Supreme Court jurisprudence. Also, there are clear connections among the effective assistance of counsel, the duty to investigate and discovery. This article explores those connections and to urge a Sixth Amendment analysis of restrictive discovery rules and describes how restrictive discovery rules block the delivery of effective assistance of counsel when defense counsel has insufficient information to investigate the case. Finally, this article summarizes the purpose of pretrial discovery in criminal cases and discusses why the three major contentions in support of restrictive discovery-likely perjury by the defendant, lack of reciprocity in the discovery process, and potential witness intimidation-lack validity. This article concludes by suggesting that the prophylactic rule of open file discovery is an appropriate remedy for the constitutional conflict between the duty to investigate and state barriers to the information necessary to carry out that duty.