The minimum surface tension and respreadability of a surfactant monolayer is limited by a two to three dimensional instability called collapse. Liquid-condensed or solid phase monolayers collapse via fracture followed by loss of material. Liquid-expanded phase monolayers collapse by solubilization into the subphase. Monolayers that retain a continuous liquid-expanded phase network surrounding islands of liquid-condensed or solid phase collapse at low surface tensions via a localized, large amplitude buckling. The buckled regions coexist with the flat monolayer, remain attached to the interface, and reversibly reincorporate into the monolayer upon expansion.