The L3 experiment was one of four large detectors on the Large Electron-Positron collider (LEP). The detector was designed to look for the physics of the Standard Model and beyond. It started up in 1989 and stopped taking data in November 2000 to make room for construction of the Large Hadron Collider (LHC). The ALICE detector now sits in the cavern that L3 used to occupy, reusing L3's characteristic red octagonal magnet.

L3 was composed of numerous subdetectors around LEP's central beamline, where electrons and positrons were made to collide. The first subdetectors out from the beamline were a silicon strip microvertex detector and a time-expansion chamber, both of which traced the paths of charged particles from the collision.

The three main outer layers of the L3 detector were an electromagnetic calorimeter (called the "BGO" – it was made of bismuth germanium oxide), a hadronic calorimeter and a muon detector. Calorimeters are made of dense material – they stop particles to measure the energy deposited. A set of scintillation counters housed between the electromagnetic and hadronic calorimeters helped to recognize and reject signals from cosmic-ray muons, energetic particles from space that could disturb measurements.

L3's outermost layer held what was then the largest magnet in the world (though this title has passed to the Compact Muon Solenoid, CMS). The L3 magnet generated a field 10,000 times stronger than the average field on the Earth's surface. Magnetic fields are often included in detectors to deflect charged particles. Physicists use the curvature of the deflection to calculate a particle's momentum.

Data from L3 helped physicists to measure W particles with unprecedented accuracy, to set limits on the possible mass of the Higgs boson, and to determine important properties of the Z boson (specifically a value called the "lineshape of the Z⁰ resonance"). The Z boson is one of the carriers of the weak force. Measuring its mass and the various ways it can decay into other particles made it possible to determine how many families of leptons (subatomic particles which do not take part in the strong interaction) exist in nature. With the help of L3, the number of lepton families was confirmed to be three.