This Special Issue is motivated by the rapid development, in recent years, of a multidisciplinary field of research and practice that has become known as Animal-Computer Interaction (ACI). The issue has two aims; the first is to acknowledge such a development, tracing its motivations in the scientific interests and concerns of the interaction design and other communities of research and practice. This is set against a broader cultural background that has seen a reassessment of the relation between humans and other animals in the face of major social, scientific and environmental transformations. The second aim of this special issue is to bring to the fore the challenges faced by ACI researchers and practitioners, but also the opportunities open to them, and to the field, and to trace a roadmap towards the development of ACI as a discipline around core aims and values. In this respect, the contributions contained in this special issue illustrate the field’s state-of-the-art and surface key questions and challenges that ACI researchers are grappling with, as well as the ways in which these might be addressed. At the same time, they highlight the gaps that still exist in the field and directions for further work that might continue to develop ACI as a discipline.