Broadening contingents of ecologists and environmental scientists have recently begun to promote ecological resilience both as a conceptual framework and as a practical goal. As some critics have noted, this growing interest has brought with it a multiplication of notions of ecological resilience. This paper reviews how and why the notion of ecological resilience has been adopted, used, and defended in ecology since its introduction by C. S. Holling in 1973. We highlight the many faces of ecological resilience, but unlike other reviewers who see these as disunified and confused, we interpret ecological resilience as an evolving, multidimensional, theoretical concept unified by its role in guiding practical response to ecological and environmental challenges. This perspective informs a review of some of the factors often recognized as favoring resilience (structural and response diversity, functional redundancy, modularity, and spatial heterogeneity); we show how the roles and relationships of these factors can be clarified by considering them in the theoretical framework of Complex Adaptive Systems (CASs).