This chapter raises the question how theory of practical knowledge can be applied to contribute to the practice and theory of validation (i.e. recognition of prior learning). The focus of the investigation is on a problem that is at the core of validation: how can assessment for validation be, at the same time, reliable and valid. In contemporary research on validation the trustworthiness of assessment is discussed in terms of validity and reliability, two ideals drawn from the social and natural sciences. Research on the recognition of prior learning suggests that the relation between reliability and validity of assessment tends to be inversely proportional. If validity increases then reliability decreases and vice versa. This leads to a problem in the practice of validation. Why? Because it puts practitioners under pressure of satisfying both ideals at the same time. Practitioners try to increase reliability by formalizing assessment procedures, though formalization tends to render invisible and exclude informal knowledge. Using an example of validation from vocational education this chapter shows how this contradiction can be overcome, or at least be eased up, by seeing the problem from the point of view of the theory of practical knowledge.