Validity is an important concept in establishing the credibility of research findings. However, the current debate about the criteria used to substantiate claims for the validity of research evidence is largely based on a set of distinctions between qualitative and quantitative methodology which are outdated and misleading. This paper argues that validity is an epistemological concept, whose application depends upon some fundamental positions taken about the nature of truth, representation and scientific methodology. There remain important, unresolved questions about the meaning and application of the concept of validity which have a crucial bearing on what criteria are used to establish the credibility of research evidence.