ALTHOUGH CONFIDENTIALITY OF INFORMANTS HAS LONG BEEN AFFIRMED IN FEDERAL LAW, IT IS NOT ABSOLUTE. GOVERNMENT WITNESSES CAN BE COMPELLED TO DISCLOSE THEIR SOURCES IF THE DEFENSE REQUIRES. HOWEVER, CONGRESS'S JENCKS ACT OF 1957 LIMITS THE TYPE OF PRETRIAL STATEMENTS TO BE GIVEN TO THE DEFENSE. INFORMANTS MUST GIVE THEIR CORRECT NAMES AND ADDRESSES AT TRIAL TO ESTABLISH CREDIBILITY ONLY IF THEIR TESTIMONY DIFFERS FROM THE DEFENDANT'S AND THE DEFENSE WISHES TO IMPEACH. WITNESSES NEED NOT BE DISCLOSED AT TRIAL IF IT CAN BE SHOWN THAT THEIR HARM WILL ENSUE. IN USING INFORMANTS TO OBTAIN SEARCH OR ARREST WARRANTS, POLICE ARE USING HEARSAY AND CONSEQUENTLY MUST SHOW IT TO BE RELIABLE. IN AGUILAR V. TEXAS, TESTS FOR DETERMINING PROBABLE CAUSE BASED ON HEARSAY WERE THAT INFORMATION PRESENTED TO A NEUTRAL MAGISTRATE MUST BE SUFFICIENT TO ESTABLISH CREDIBILITY OF THE INFORMANT AND HIS INFORMATION. THREE TYPES OF POLICE INFORMANTS INCLUDE CRIMINALS, CITIZENS, AND POLICE OFFICERS. THE CRIMINAL USUALLY INFORMS FOR SOME POLICE FAVOR, BUT CAN CREATE RELIABILITY PROBLEMS. USUALLY, CRIMINAL INFORMANTS MUST HAVE PROVIDED RELIABLE INFORMATION IN THE PAST OR INFORMATION IMPLICATING THEMSELVES. CITIZEN AND POLICE OFFICER INFORMANTS USUALLY PRESENT FEW RELIABILITY PROBLEMS, BUT MAGISTRATES MUST BE GIVEN SUFFICIENT BACKGROUND INFORMATION TO ASCERTAIN RELIABILITY IF THE INFORMANT IS NOT TO BE DISCLOSED. EXAMPLES ARE GIVEN OF SUFFICIENT AND INSUFFICIENT CORROBORATION OF INFORMANT'S INFORMATION FOR PROBABLE CAUSE ARREST.