Reagan & Iran Contra, from "The American President" website, I quote:

"The investigation by a special Senate committee—presented on national television —revealed that North and John Poindexter (Reagan's national security adviser) had routinely lied to Congress, destroyed official documents, misled cabinet officials, and practiced a foreign policy agenda characterized by deception and a flagrant disregard for the truth. Even though his role in the affair remained suspect, President George Bush eventually pardoned six key players in the Iran-Contra scandal in December 1992, after he had lost his reelection bid to William Clinton. Among those pardoned was Secretary of Defense Caspar Weinberger, who was scheduled to be tried for perjury.

Although the scandal damaged the Reagan administration politically, there emerged no serious call for his impeachment. Unlike in the Watergate cover-up, no "smoking gun" linked Reagan directly to the affair. Although Colonel North always believed that Reagan had authorized the covert action, he refused to acknowledge having ever directly talked with the president about it. When John Poindexter testified under oath that he had purposefully kept Reagan out of the loop in order to protect him, the link to Reagan could not be proved. The one person who might have implicated Reagan, CIA Director William Casey, died from cancer before being subpoenaed. And President Reagan was never called to testify under oath. When asked unofficially about what he knew, Reagan professed not to remember ever knowing anything about channeling funds from arms sales to the Contras. He did not deny knowing about it, just that he could not remember.

The call for Reagan's head also was undermined by a speech he delivered from the Oval Office on March 4, 1987. In a vintage Reagan performance, the president acknowledged that his administration, unbeknownst to himself, had traded arms for hostages. He said that the entire operation had happened because of patriotic staffers who misinterpreted his policy in their zeal to defeat terrorism, to liberate American hostages, and to aid the freedom fighters in Nicaragua. Most importantly, Reagan refused to make excuses for himself and bluntly admitted that the whole affair had been a serious mistake. Just prior to the speech, a special investigative committee appointed by Reagan and led by former Senator John Tower of Texas had determined that the president—though guilty of managerial negligence—was not directly involved in the transfer of funds to the Contras, which was considered the more serious offense."

http://www.americanpresident.org/kotrain/courses/RR/RR_Foreign_Affairs.htm