Human spoken interaction is a phenomenon with which we are all too familiar from our everyday experience; we take it for granted as we are immersed in it. In this book we view human spoken interaction afresh as both an object of research and as a remarkable phenomenon worthy of wonder. We show how it functions as a complex adaptive system in similar ways to other life-related systems, how it enables us to communicate with each other and perform a vast range of intricate social actions. We are not looking at how language functions as a complex adaptive system, which has been the focus of other studies (e.g., N.C. Ellis and Larsen-Freeman 2009b). Rather, we align with Levinson’s (2006, 2023) conception of the ‘interaction engine’ as the more fundamental, more primal system which evolved before language and languages and which is acquired by infants before language(s).