"Article I, § 2 of the United States Constitution provides the President the authority 'to grant Reprieves and Pardons for Offenses against the United States, except in Cases of Impeachment.' While the text makes it clear that the pardon power is limited to federal offenses ('offenses against the United States') and cannot be used to avoid impeachment, the terse, one-sentence provision offers little other guidance on the scope of the President's pardon authority. The Supreme Court has stated that the President's pardon power is near plenary, but the exercise of this authority may occasionally prompt questions regarding the power's compatibility with notions of fairness and the rule of law. Recently, some Members of Congress (see, e.g., here) and legal observers have raised or opined upon various, oftentimes difficult, legal questions pertaining to the pardon power, including whether the President can issue 'prospective' pardons; whether the President can pardon himself; and the extent to which Congress can regulate or respond to the exercise of the President's pardon authority. This Sidebar provides a general overview of the pardon power and briefly addresses a few frequently asked legal questions concerning its scope and application."
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html