In the sport of association football, a defender is an outfield player whose primary role is to stop attacks during the game and prevent the opposition from scoring.

Defenders fall into four main categories: centre-backs, full-backs, sweepers, and wing-backs. The centre-back and full-back positions are most common in modern formations. The sweeper and wing-back roles are more specialised, often limited to certain formations dependent on the manager's style of play and tactics.

The centre-back (also known as a central defender or centre-half, as the modern role of the centre-back arose from the centre-half position) defends in the area directly in front of the goal and tries to prevent opposing players, particularly centre-forwards, from scoring. Centre-backs accomplish this by blocking shots, tackling, intercepting passes, contesting headers and marking forwards to discourage the opposing team from passing to them. Centre-backs are often tall and positioned for their ability to win duels in the air. In the modern game, most teams employ two or three centre-backs in front of the goalkeeper. The 4–2–3–1, 4–3–3, and 4–4–2 formations all use two centre-backs.

In possession of the ball, centre-backs are generally expected to make long and pinpoint passes to their teammates, or to kick unaimed long balls down the field. For example, a clearance is a long unaimed kick intended to move the ball as far as possible from the defender's goal. Due to the many skills centre-backs are required to possess in the modern game, many successful contemporary central-defensive partnerships have involved pairing a more physical defender with a defender who is quicker, more comfortable in possession and capable of playing the ball out from the back; examples of such pairings have included Carles Puyol and Gerard Pique for Barcelona and Spain, David Luiz, Gary Cahill, John Terry and Ricardo Carvalho with Chelsea, Sergio Ramos, Raphaël Varane or Pepe with Real Madrid, Diego Godín and José María Giménez with Atlético Madrid and Uruguay, Virgil van Dijk with Liverpool, Nemanja Vidić and Rio Ferdinand with Manchester United, or Giorgio Chiellini, Leonardo Bonucci, and Andrea Barzagli with Juventus.

Under normal circumstances, centre-backs are unlikely to score goals. However, when their team takes a corner kick or other set pieces, centre-backs may move forward to the opponents' penalty area; if the ball is passed in the air towards a crowd of players near the goal, then the heading ability of a centre-back is useful when trying to score. In this case, other defenders or midfielders will temporarily move into the centre-back positions.

There are two main defensive strategies used by defensive lines: the zonal defence, where each centre-back covers a specific area of the pitch; and man-to-man marking, where each centre-back has the job of tracking a particular opposition player. In the now obsolete man–to–man marking systems such as catenaccio, as well as the zona mista strategy that later arose from it, there were often at least two types of centre-backs who played alongside one another: at least one man–to–man marking centre-back, known as the stopper, and a free defender, which was usually known as the sweeper, or libero, whose tasks included sweeping up balls for teammates and also initiating attacks.

The sweeper (or libero) is a more versatile centre-half who "sweeps up" the ball if an opponent manages to breach the defensive line. This position is deemed more fluid than that of the other defenders who man-mark their designated opponents. Because of this, it is sometimes referred to as libero, which is Italian for "free".

Austrian manager Karl Rappan is thought to be a pioneer of this role, when he incorporated it into his catenaccio or verrou (also "doorbolt/chain" in French) system with Swiss club Servette during the 1930s, deciding to move one player from midfield to a position behind the defensive line, as a "last man" who would protect the back-line and start attacks again. As coach of Switzerland in the 1930s and 1940s, Rappan played a defensive sweeper called the verrouilleur, positioned just ahead of the goalkeeper.