This lecture covers circular accelerators, colliders, luminosity calculation, beam-beam force derivation, linear and Gaussian beam-beam forces, different types of collisions, and the LHC luminosity reach. It explains the beam-beam parameter, linear approximation, head-on and long-range interactions, detuning with amplitude, and the complications in collider operations. The geometric reduction factor, crossing angle operation, and the effects of long-range interactions are also discussed, along with observations in hadron colliders and the beam-beam limit in lepton colliders.