SOCIAL WORKERS WILL OFTEN SAY that they use systems theories in their practice. People are inextricably linked to their environments, and theories that call on systems of experience and interaction tend to make logical and practical sense to workers in daily practice. Systems theories help us to think about these interactions between people and their social and physical environments, and they help us to understand how change can occur through the use of ecosystem interventions. In chapter 1, we talked about how social work theory helps us to explain human experience and how people and their environments change. For the profession, this has often meant engaging in a search for a single unifying theoretical approach. This was particularly pursued during the first half of the twentieth century when social work was criticized for being more of a ‘cause’ than a profession, particularly given that it had no scientific theoretical base (Flexner 1915; Austin 1983; Leighninger 1987). This seems an unusual critique these days, as no profession would be able to argue that it adhered to a particular causal explanation of the world and its role as a profession within it. In those days, however, the pressure was on to justify professions according to their scientific base. And from the second half of the twentieth century systems theory looked as if it might just be the one needed to save the social work profession by providing this elusive unifying approach. In recent decades we have reduced our expectations that systems theory will save the profession, as we have gained appreciation of the value of multiple perspectives in response to complex need. Systems theory has nevertheless continued to be important to social work thinking and practice as it provides a foundation for much of social work's understanding of human adaptation and coping in the face of adversity.