Child well-being is regarded as an important part of early childhood education but little is known about the meaning of well-being and how it is operationalised in a curriculum context. The paper examines how practitioners in Wales assess the well-being of 3- to 7-year-olds. The case study involves two schools and the paper discusses findings from six focus groups and fourteen interviews. Practitioners are uncertain about how to assess child well-being and therefore draw upon what they do in other subject areas and use criterion-referenced assessment. Practitioners also report various difficulties in assessing well-being which suggests that placing well-being in curriculum policy is problematic. Findings indicate that well-being is understood as an abstract concept where practitioners attempt to transform it into a concrete form in order to assess it. Questions linking to rationale, concept, enactment and assessment need to be discussed when considering well-being as a subject area.