was the nickname given to the front four of the famous defensive line of the American football team Pittsburgh Steelers during their 1970s dynasty years. This defense was the backbone of the Steelers dynasty, which won 4 Super Bowls (IX, X, XIII, and XIV). Over the years, the nickname evolved into a reference to the entire defensive squad.

 In the 1976 season, the Steelers' defense was a juggernaut, arguably the greatest defense of all time. After the Steelers started 1-4 and lost their quarterback, Terry Bradshaw, the Steelers' defense took over for the remaining nine games. The Steelers recorded five shutouts, including three in a row, during this stretch. During this nine-game stretch the Steelers allowed a total of two touchdowns (in the same game) and five field goals. The defense allowed an average 3.1 points per game and the team had an average margin of victory of 22 points. Eight of the Steelers' starting eleven defensive players were selected for the Pro Bowl that year. Four would be selected to the Hall of Fame.

The Steel Curtain's famed front four were: 

• #78 Dwight White - Defensive End, 1971-1980
• #63 Ernie Holmes - Defensive Tackle 1972-1977
• #75 "Mean" Joe Greene - Defensive Tackle 1969-1981
• #68 L.C. Greenwood - Defensive End 1969-1981

Source: http://en.wikipedia.org/wiki/Steel_Curtain