Ecological systems theory is concerned with the interaction and interdependence of individuals with their surrounding systems and encourages social workers to take a holistic view by assessing how individuals affect and are affected by such physical, social, political, and cultural systems. The origins of ecological systems theory was derived from the separate frameworks of ecological theory, systems theory, and Bronfenbrenner’s bioecological systems theory. The modern use of the ecological systems theory is based on five assumptions that examine the interaction and interdependence of systems and how the assessment of one system cannot be done in isolation, but, rather, must take into account how the systems affect and are affected by each other. Ecological systems theory is useful in the assessment stage of social work practice as it can be a framework in which to assess a client from a holistic perspective, particularly through the use of ecomaps. Based on the social worker’s assessment, ecological systems theory can be useful in pointing the social worker to the most appropriate system in which to intervene.