3-4

In American football, the 3–4 defense is a common defensive alignment consisting of three down linemen and four linebackers. It is a called a "base defense" because it is the default defensive alignment used on "base downs" (1st and 2nd downs). However, defenses will readily switch to other defensive alignments (such as a dime) as circumstances change. Alternately some defenses use a 4-3.

The 3–4 defense incorporates three defensive linemen - two defensive ends and one nose tackle, who line up opposite the other team's offensive line. Those three players are responsible for engaging the other team's offensive line, allowing the four linebackers to either rush the quarterback or drop back into coverage, depending on the situation. While the role of the defensive linemen is fairly consistent, the linebackers allow for the flexibility and versatility of the 3-4 scheme, and give defensive coaches nearly limitless options to confuse the other team's players and coaches. Depending on the situation, any number of linebackers can blitz, fake a blitz, "spy" the quarterback or running back, or cover receivers. In key situations, a rush linebacker may be sent to cover the flat on the opposite side of the blitzing defensive back; this is called a "zone blitz".

After becoming the predominant defensive alignment in the late 1970s-early 1980s, the 3–4 defense declined in popularity over the next two decades, but experienced a resurgence in the 2000s among both professional and college football teams. As of 2017, NFL teams that regularly incorporate the 3–4 defensive alignment scheme as a base include the Los Angeles Chargers, Green Bay Packers, Baltimore Ravens, Arizona Cardinals, Indianapolis Colts, Kansas City Chiefs, New York Jets, Pittsburgh Steelers, Washington Redskins, Denver Broncos, Los Angeles Rams, Tennessee Titans, Houston Texans, and the Chicago Bears, who used the 3-4 as their base defense for the first time in 2015. The Philadelphia Eagles ran the 3-4 from 2013 to 2015 but will switch back to the 4-3 for the 2016 season. The New England Patriots returned to a 4–3 defensive front at the end of the 2011 season.

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