This lecture provides an overview of systems theory and the concepts of physiological and psychological stress. As discussed in earlier lectures, the ecological paradigm and systems theory developed in response to narrower, deterministic explanations of environmental influences on human behavior and well-being. In ecological systems analyses, the degree of fit or congruence achieved by people and their surroundings depends on a variety of context-specific circumstances, such as spatial arrangements and staffing levels of behavior settings, personality orientations, social and cultural norms. When the levels of fit between people and their surroundings are low, physiological and psychological stress can arise—for example, in highly demanding or constraining environments. Examples of systems processes are presented at different levels of analysis ranging from individuals, small groups, and entire populations. The distinction between deviation-countering and deviation-amplifying systems is also discussed.