The free energy principle (FEP) is a formulation of the adaptive, belief-driven behaviour of self-organizing systems that gained prominence in the early 2000s as a unified model of the brain [1, 2]. Since then, the theory has been applied to a wide range of biotic phenomena, extending from single cells and flora [3, 4], the emergence of life and evolutionary dynamics [5, 6], and to the biosphere itself [7]. For our part, we have previously proposed that the FEP can be integrated with Tinbergen’s seminal four questions in biology to furnish a multiscale ontology of living systems [8]. We have also explored more specific applications, eg, to the evolution and development of human phenotypes [9–11], sociocultural cognition, behaviour, and learning [12, 13], as well as the dynamic construction of environmental niches by their denizens [14, 15]. Despite such contributions, the capacity of the FEP to extend beyond the human brain and behaviour, and to explain living systems more generally, has only begun to be explored. This begs the following questions: Can the FEP be applied to any organism? Does it allow us to explain the dynamics of all living systems, including large-scale social behaviour? Does the FEP provide a formal, empirically tractable theory of any complex adaptive system, living or not? With such questions in mind, the aim of this Special Issue was to showcase the breadth of the FEP as a unified theory of complex adaptive systems, biological or otherwise. Instead of concentrating on the human brain and behaviour, we welcomed contributions that applied the FEP to other complex adaptive systems, with the hope of exemplifying the extent of …