The idea that validity should be considered a property of inferences, rather than of assessments, has developed slowly over the past century. In early writings about the validity of educational assessments, validity was defined as a property of an assessment. The most common definition was that an assessment was valid to the extent that it assessed what it purported to assess and this definition is still in widespread use. The idea that construct interpretations are at the heart of validity argument and that therefore construct definition is essential to effective assessment has also now become widely accepted, at least within the measurement community. In this article, the author traces one strand in the development of the theory of validity--the increasing importance attached to the role of constructs in validating educational assessments--and shows how, ensuring that the primary focus is on the construct of interest, rather than the assessment itself, can bring some greater clarity to a number of debates, especially in the area of equity in assessment. The author discusses, in some detail, three particular arenas of assessment: (1) testing for admission to higher education; (2) the rise, and fall, of measures involving constructed-response items; and (3) the assessment of students with special educational needs. Within each of these areas, the author explores the interplay of issues of technical adequacy with equity and shows how attention that has in the past been directed at the adequacy of the assessments might be more fruitfully directed toward the construct of interest. (Contains 1 table.)