The aim of this paper is to examine conceptual issues that challenge the development of valid and useful measures of children’s participation. Ambiguities in the current definition of participation in the International Classification of Functioning, Disability and Health (ICF) are examined along with their implications for developing valid measures for children and youth. Variations in how the participation construct is operationalized challenge one’s ability to develop a coherent body of knowledge about children’s participation and the factors that influence it. Given current variations in how participation is being defined, both developers and users of measures of participation need to be explicit about the definition of participation that a particular measure represents and the inferences that can be drawn from the scores.