The book provides a new conceptual framework to explain the interaction of complex systems, specifically humans and their environment. It proposes that human physiology and the environment do not “connect” with each other in a direct, unidirectional manner, like a beaker pouring water into a cup. Rather, the authors propose the Biodynamic Interface Conjecture with the central axiom that complex systems cannot interact directly or exist in isolation due to temporally embedded functional interdependencies within and between systems. The authors propose that human physiology and the environment contribute to the formation of an interface, and by doing so they give rise to an intermediary that guides the interaction by letting some influences pass between the systems while restricting others. This proposition counters many structural approaches that assume that complex systems, such as the environment and humans, can transfer information directly between them while remaining discrete entities. Although developed for environmental health sciences, the conjecture has broader implications for the study of complex system interactions across various levels of organization, and the central role of time and temporal dynamics in system-to-system information exchange. This conjecture also argues against causal paradigms that (incorrectly) assume that systems are distinct entities interacting directly and ignore boundary conditions, and organizational levels, and complexity inherent in biological and environmental systems.