Background Best interests is a ubiquitous principle in medical policy and practice, informing the treatment of both children and adults. Yet theory underlying the concept of best interests is unclear and rarely articulated. This paper examines bioethical literature for theoretical accounts of best interests to gain a better sense of the meanings and underlying philosophy that structure understandings. Methods A scoping review of was undertaken. Following a literature search, 57 sources were selected and analysed using the thematic method. Results Three (...) themes emerged. The first placed best interests within the structure of wider theory, noting relationships with consequentialism, deontology, prudential value theory, rights and political philosophy. The second mapped a typology of processes of decision-making, among which best interests was ambiguously positioned. It further indicated factors that informed best interests decision-making, primarily preferences, dignity and quality of life. The final theme considered best interests from a relational perspective. Conclusions Characterisation of best interests as strictly paternalist and consequentialist is questionable: while accounts often suggested a consequentialist basis for best interests, arguments appeared philosophically weak. Deontological accounts, found in law and Kantianism, and theories of political liberalism influenced accounts of best interests, with accounts often associating best interests with negative patient preferences. There was much more emphasis on negative interests than positive interests. Besides preference, factors like dignity and quality of life were held to inform best interests decisions, but generally were weakly defined. To the extent that preferences were unable to inform decision making, decisions were either made by proxy authority or by an intersubjective process of diffuse authority. Differing approaches reflect bifurcations in liberal philosophy between new liberalism and neo-liberalism. Although neither account of authority appears dominant, bias to negative interests suggests that bioethical debate tends to reflect the widespread ascendancy of neo-liberalism. This attitude was underscored by the way relational accounts converged on private familial authority. The visible connections to theory suggest that best interests is underpinned by socio-political trends that may set up frictions with practice. How practice negotiates these frictions remains a key question. (shrink)