The process of giving official acknowledgment to formal, informal and non-formal prior learning is commonly labelled as assessment, accreditation or recognition of prior learning (APL), representing a practice that is expanding in higher education in many countries. This paper focuses specifically on the assessment part of APL, which undoubtedly is central to the whole process, through a review of research in this area and an analysis of the reviewed studies from a validity perspective. The research reviewed (published 1990-2007) is categorised into empirical as well as more theoretically oriented publications, with a quantitative dominance of the latter. According to the validity analysis, a majority of the studies conducted in this area relate to the evidential basis of test interpretation and use, primarily providing theoretical rationales and theories for a variety of practices. The consequential basis of test interpretation and use has not been studied to any larger extent, resulting in a lack of both theoretical and empirical studies dealing with this aspect of validity. (Contains 1 table.)