The Rat Pack is the nickname given to a group of popular entertainers most active between the mid-1950s and mid-1960s. Its most famous line-up featured Frank Sinatra, Dean Martin, Sammy Davis, Jr., Peter Lawford and Joey Bishop, who appeared together in films and on stage in the early-1960s.Despite its reputation as a masculine group, the Rat Pack did have female participants, including movie icon s Shirley MacLaine, Lauren Bacall, and Judy Garland.

The name "Rat Pack" was first used to refer to a group of friends in Hollywood informally organized around Humphrey Bogart and including the young Frank Sinatra. Several explanations have been offered for the famous name over the years. According to one version, the group's original "Den Mother," Lauren Bacall, after seeing her husband and his friends return from a night in Las Vegas, said words to the effect of "You look like a goddamn rat pack. "Rat Pack" may also be a shortened version of "Holmby Hills Rat Pack," a reference to the home of Judy Garland and husband Sid Luft, which served as a regular hangout.The name may also refer to the belief that an established pack of rats will belligerently reject an outsider who tries to join them.Judy Garland's daughter, Lorna Luft, tells the story that a certain gossip columnist (probably Hedda Hopper) wanted to be invited to the group's parties. The group didn't want their private parties becoming the subject of the writer's next column, and so the columnist was never invited. Later, she was said to have written about "that rat pack in Holmby Hills" which Garland found incredibly funny. Garland later had stick pins made for the group in the shape of rats with rubies for eyes.Thus, the "Rat Pack" was born.

According to Stephen Bogart, the original members of the Holmby Hills Rat Pack were Sinatra (pack master), Garland (first vice-president), Bacall (den mother), Luft (cage master), Bogart (rat in charge of public relations), Swifty Lazar (recording secretary and treasurer), Nathaniel Benchley (historian), David Niven, Katharine Hepburn, Spencer Tracy, George Cukor, Michael Romanoff and Jimmy Van Heusen.In his autobiography The Moon's a Balloon, actor David Niven confirms that the Rat Pack originally included him but not Sammy Davis or Dean Martin.

The 1960s version of the group included Frank Sinatra, Dean Martin, Sammy Davis, Jr., Joey Bishop, Peter Lawford (brother-in-law of John F. Kennedy), and for a brief stint, Norman Fell. Angie Dickinson, Juliet Prowse, and Shirley MacLaine were often referred to as the "Rat Pack Mascots", a title which reportedly made these ladies feel like "one of the boys". The post-Bogart version of the group was reportedly never called that name by any of its members — they called it the Summit or the Clan. "The Rat Pack" was a term used by journalists and outsiders, although it remains the lasting name for the group.

As a result of Lawford's relation to Kennedy and Sinatra's connections to the Mafia, and the role the group played in campaigning for Kennedy and the Democrats, the Rat Pack had not only influence in entertainment and social circles but some influence politically as well. Sinatra expected that he would be part of Kennedy's circle after the election but was excluded, which in turn led to Peter Lawford's exclusion from the group after 1962. Lawford's role in Robin and the Seven Hoods was given to Bing Crosby and spiced up with several songs. (It wasn't the first time Sinatra had treated a Rat Packer that way; Davis's role in Never So Few was given to Steve McQueen when Sinatra and Davis had a temporary falling-out.)

The Rat Pack often performed in Las Vegas, Nevada, and were instrumental in the rise of Las Vegas as a popular entertainment destination. They played an important role in the desegregation of Las Vegas hotels and casinos in the early 1960s. Sinatra and the others would refuse to play in or patronize those establishments that would not give full service to African American entertainers including Davis. Once Rat Pack appearances became popular and the subject of media attention, the Las Vegas properties were forced to abandon segregation-based policies.

Sinatra and friends had no idea this band of five would make entertainment history. The group was remarkable for its upbeat entertainment style and smooth musical and comedy routines, many of which were ad-libbed. Davis said when Sinatra called the initial gathering of the Rat Pack, U.S. President Dwight D. Eisenhower, French President Charles de Gaulle, and Soviet leader Nikita Khrushchev were planning a Paris Summit Conference. Not to be outdone, Sinatra observed, "We'll have our own little Summit meeting." The Vegas Summit did not draw diplomats, but it did draw high rollers, VIPs, celebrities, and entertainment buffs, who responded by the thousands.

Often, when one of the members was scheduled to give a performance, the rest of the Pack would show up for an impromptu show, causing much excitement amongst audiences resulting in return visits. They sold out almost all of their appearances, and people would come pouring into Las Vegas, sometimes sleeping in cars and hotel lobbies when they could not find rooms, just to be part of the Rat Pack's entertainment experience. The marquees of the hotels at which they were performing as individuals might read "DEAN MARTIN - MAYBE FRANK - MAYBE SAMMY."

Although the Rat Pack members remained close (with the exception of Peter Lawford), the Rat Pack began to fade in popularity with the rise of the 1960s counterculture, which sent their form of sophisticated "Establishment" entertainment into decline. While its individual members remained hugely popular with the public, the Rat Pack, as such, had ceased to exist by the end of the 1960s.

Martin and Davis appeared together in the movie Cannonball Run, and later were joined by Sinatra in the movie Cannonball Run II. This would be the last time that the three would appear in a movie together.

Peter Lawford died on December 24, 1984 of cardiac arrest complicated by kidney and liver failure, at the age of 61. Sammy Davis, Jr. died at the age of 64 on May 16, 1990, of complications from throat cancer. Dean Martin died at home on Christmas morning 1995, aged 78. Frank Sinatra died on on May 14, 1998, at the age of 82. Joey Bishop, the last surviving and longest-lived (89) Rat Pack member, died on October 17, 2007.