Liminality was described more than 20 years ago as a major category explaining how cancer is experienced. Since then, it has been widely used in the field of oncology research, particularly by those using qualitative methods to study patient experience. This body of work has great potential to illuminate the subjective dimensions of life and death with cancer. However, the review also reveals a tendency for sporadic and opportunistic applications of the concept of liminality. Rather than being developed in a systematic way, liminality theory is being recurrently 're-discovered' in relatively isolated studies, mostly within the realm of qualitative studies of 'patient experience'. This limits the capacity of this approach to influence oncological theory and practice. In providing a theoretically informed critical review of liminality literature in the field of oncology, this paper proposes ways of systematizing liminality research in line with a processual ontology. In so doing, it argues for a closer engagement with the source theory and data, and with more recent liminality theory, and it sketches the broad epistemological consequences and applications.