There have been several attempted assassinations on U.S. Presidents throughout history. Here are some of the notable cases:
Andrew Jackson (1835): The first recorded attempt on a sitting U.S. President occurred when Richard Lawrence attempted to shoot Andrew Jackson. Both of Lawrence's pistols misfired, and Jackson attacked him with his cane.
Abraham Lincoln (1861 & 1865): There was an earlier, less-known attempt on Lincoln's life in 1861, before his inauguration, involving a plot to kill him on his way to Washington, D.C. However, the most famous attempt was the successful assassination by John Wilkes Booth in 1865.
Theodore Roosevelt (1912): Although not a sitting President at the time, Roosevelt was shot by John Schrank during a campaign event. The bullet lodged in his chest, but Roosevelt continued his speech for over an hour before seeking medical attention.
Franklin D. Roosevelt (1933): Giuseppe Zangara attempted to assassinate FDR just before his inauguration. The attempt failed, but Chicago Mayor Anton Cermak was killed.
Harry S. Truman (1950): Two Puerto Rican nationalists attempted to storm Blair House, where Truman was staying. A shootout ensued, and one of the attackers was killed.
John F. Kennedy (1963): While there were earlier attempts, the most notable was the successful assassination in Dallas, Texas, by Lee Harvey Oswald.
Richard Nixon (1974): Samuel Byck planned to hijack a plane and crash it into the White House in an attempt to kill Nixon. The plan failed when he was shot and killed before the plane took off.
Gerald Ford (1975): There were two separate attempts on Ford's life within three weeks. Lynette "Squeaky" Fromme attempted to shoot him in Sacramento, California, but her gun misfired. Later, Sara Jane Moore tried to assassinate Ford in San Francisco but was thwarted by a bystander.
Jimmy Carter (1979): Raymond Lee Harvey was arrested for plotting to shoot Carter, but the attempt was foiled before it could be carried out.
Ronald Reagan (1981): John Hinckley Jr. shot Reagan outside a Washington, D.C., hotel. Reagan was wounded but survived.
George H.W. Bush (1993): There was an alleged plot by Iraqi intelligence to assassinate Bush during a visit to Kuwait. The plot was foiled.

These are some of the most notable assassination attempts, though there have been other threats and minor incidents involving U.S. Presidents over the years.