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) much-less-than 1) surface-bulk exchange kinetics are rapid; the interface releases its adsorbed molecules before diffusion can disturb the bulk density profile. Surface coverage GAMMA relaxes exponentially in a time Q-1. Exchange kinetics for "strong" systems (Qt(h) much-greater-than 1) are by contrast sufficiently slow that diffusion-controlled effects develop; the bulk is strongly disturbed near the surface with GAMMA 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 t approximately 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.