The study of ecosystems in action, by measuring ecosystem recovery from disturbance, resistance to alterations, and the reversibility of ecosystem changes, highlights features of natural communities that contribute to resilience. Examples from marine intertidal and subtidal communities document the importance of species redundancy and complementarity in resistance and recovery, and they also show why recovery potential and resistance can differ from place to place within the same ecosystem. Whether a change is considered reversible may depend on the timescale of interest, and on whether fundamental new ecological processes have taken hold after a disturbance. By focusing on recovery, resistance, and reversibility as key components of resilience, marine ecologists have provided a much-needed empirical database about the response of the living world to human-mediated change.