The article first traces the history of informal immunity grants and examines the prosecutor's discretion in the choice of whom to immunize or treat leniently in return for cooperation. Ways of monitoring this discretion are also discussed. Other issues addressed are whether the testimony admitted under cooperation agreements may infringe on a defendant's right to a fair trial and whether enforcement of cooperation agreements by the State may impose an unfair burden on the cooperating witness. The chief dangers of cooperation agreements are the improper or imprudent selection of the beneficiaries of informal immunity or lenient bargains; the presentation of unacceptably tainted or suspect testimony against defendants; and agreements with cooperators that may impose unconscionable obligations on cooperators, confer unacceptable license to commit future crimes, or unreasonably forgive their past crimes. Another danger consists of the possibility of vindictive or excessively harsh retaliations against cooperators who, in a prosecutor's opinion, have not complied with their obligations under the agreement. Even given these dangers, the cooperation agreement may be a necessary instrument for the prosecution of certain dangerous and elusive criminals. Its risks and dangers must be addressed through administrative rules. 246 footnotes