This article presents a conceptual model for occupational therapy: the ecological systems model. The developing individual is viewed as an open system interacting with the environment. Both humans and their environment are interconnected, joined in shaping each other. States of health and illness can be seen as reflections of ecological adaptation. Function or dysfunction is evaluated in terms of persons’ effectiveness in achieving their own goals, for their own quality of life, in their interactions in the ecosystem. If performance of necessary tasks and roles is effective in achieving those goals and in harmony with the needs and goals of the external systems, then behavior is functional.