The prosecutor is the representative not of an ordinary party to a controversy, but of a sovereignty whose obligation to govern impartially is as compelling as its obligation to govern at all, and whose interest, therefore, in a criminal prosecution is not that it shall win a case, but that justice shall be done. As such, (s)he is in a peculiar and very definite sense the servant of the law, the two-fold aim of which is that guilt shall not escape or innocence suffer. (S)He may prosecute with earnestness and vigor -- indeed, (s)he should do so. But, while (s)he may strike hard blows, (s)he is not at liberty to strike foul ones. It is as much (her)his duty to refrain from improper methods calculated to produce a wrongful conviction as it is to use every legitimate means to bring about a just one.
- Berger v. United States, 295 U.S. 78 (1935)