Article II, §2 grants the President sole authority to "grant Reprieves and Pardons for Offences against the United States, except in Cases of Impeachment." The power is essentially unreviewable: the Supreme Court has consistently held it is a core executive function.
Recent pardons — preemptive pardons before charges, pardons of close associates, self-pardon hypotheticals — have prompted reform proposals: requiring DOJ pardon-attorney review, banning self-pardons by constitutional amendment, prohibiting pardons of close family or campaign aides, and creating notification requirements for Congress.
Defenders argue the pardon power is a constitutional safety valve and judicial-system check. Critics argue uncontrolled use undermines accountability and rule of law.