Numerous theories have been developed to conceptualize the nature, processes, factors involved, and outcomes of exposure to highly stressful events. These theories vary in complexity, breadth, and the frameworks on which they rely. Some are comprehensive and others have a relatively narrow focus. Most came from psychobiology, psychology, sociology, psychiatry, social work, and anthropology. This chapter presents a review of leading theories that have informed the understanding of stress and its aftermath in individuals, families, and communities. It is divided into two parts. The first reviews theories with individual focus and the second theories with systemic focus, particularly in relation to families. This distinction is somewhat artificial because some theories were originally developed relative to individuals and expanded to families whereas others developed as universal, all-encompassing frameworks that apply to individuals and systems of all sizes. To help the reader navigate this complex terrain, theories are presented according to their main focus and, when applicable, both individual and systemic versions are introduced.