Play and ritual, as usually defined, seem to be disparate phenomena, one focused on freedom and flexibility, the other on formality and rigidity. In actuality, they have many common elements, and these will be explored from a comparative perspective grounded in ethology, evolution and play theory. The description and recognition of play in diverse species and contexts, along with its origins and evolution, are discussed along with the origin of rituals in both everyday life and communal activities in human and non-human animals. The early history of linking ritualization as an important process of symbolic communication in animal courtship, dispute resolution and other ethotypic phenomena, with parallels in human cultural activities, can be traced back to the seminal papers by Oskar Heinroth and Julian Huxley in 1911 and 1914 respectively, which first described ethological ritualization. These early papers also drew many comparisons between avian and human displays and accompanying psychology, but these were lost as behaviourism flourished in comparative psychology and cultural anthropology became suspicious of evolutionary approaches to human cultural practices. In the interim, many compelling comparative data were acquired, some of which will be reviewed, along with preliminary modelling of evolutionary scenarios using comparative data. Both play and ritual are products of individual development and historical processes and a relatively recent Christian ritual with playful traits is given as an example. Today, with the development of dual inheritance theory, gene-culture co-evolution and the growing and rich biological, anthropological and archaeological data capable of being analyzed with modern behavioural, evolutionary and phylogenetic methods and theory, the intellectual landscape is rapidly changing. Thus, intriguing and important opportunities for collaboration, synthesis and extension across many areas of human life, including spirituality and religion, are now compelling. INTRODUCTION Play and ritual are universal in human behaviour and certainly both are common in non-human species as well. Play, however, is not expressed only through overt behaviour, but also takes place in our imagination, thinking and planning (Burghardt 2005). Is it reasonable to postulate that we can collect relevant data and develop models for the transition from behavioural play to mental or ‘interior’ play that then foster the distinctive human imagination and creative processes we find applied to both natural and supernatural phenomena? Do rituals, as does play, encapsulate and expand our mental lives?. © Cambridge University Press 2018.