The theory presented in this book is grounded in a systems perspective, based on which a set of assumptions, axioms, theorems, and a model are explicated. It emphasizes the stress-adaptation-growth dynamics of cross-cultural experiences, which bring about cultural strangers’ gradual transformation toward increased functional fitness in the host milieu. Effective intrapersonal (cognitive, affective and behavioural) and social (interpersonal and mass) communication activities are theorized as being at the heart of successful adaptation- along with individual background characteristics and host environmental conditions.