The adsorption-desorption kinetics of surfactant molecules at liquid surfaces are studied. We find bulk-surface systems are naturally classified as either "strongly adsorbing" or "weakly adsorbing," depending on the ratio of the desorption time Q -1 to the adsorption depth diffusion time t h . In "weak" systems ( Qt h ⪡ 1) surface-bulk exchange kinetics are rapid; the interface releases its adsorbed molecules before diffusion can disturb the bulk density profile. Surface coverage Γ relaxes exponentially in a time Q -1. Exchange kinetics for "strong" systems ( Qt h ⪢ 1) are by contrast sufficiently slow that diffusion-controlled effects develop; the bulk is strongly disturbed near the surface with Γ relaxing algebraically in a time t h . Relaxation of surface density inhomogeneities also proceeds very differently in the two cases. Of particular interest is the "anomalous" (non-Fickian) relaxation in the strong case; surface perturbations of wavevector k T relax in a time t̃ ∼ 1/ k T. We find that in strong systems the surface furnishes bulk diffusive dynamics with absorbing (reflecting) boundary conditions for t < t h ( t > t h ), while for weak systems boundary conditions are always reflecting. We discuss dynamical correlation functions which reflect the above behaviors and are in principle available through scattering measurements.