This paper reports findings from a national wide research project in Norway called "Research on individual assessment in schools" (FIVIS), with the main purpose to gain knowledge of how assessment stimulates learning and what characterizes school practices and classroom practices when assessment is used as a tool for learning.The project has drawn a picture of how assessment practices in Norwegian schools develop in an interaction between centrally set policies and theories and local interpretations and understandings at municipality level, at school level and at classroom level in a complex landscape of the Norwegian school system. Assessment practices are in this perspective constructed in a complex interplay between governmental intentions, local understandings and interpretations in schools. Validity can be seen as a threshold concept in developing assessment literacy. The FIVIS project shows that an awareness of validity and validity chains is a good concept for judging the quality of the work on assessment, whether in the interpretation of the learning objectives, the local work with the curricula, everyday conversations about learning between teachers and students or the use of standardized tests or procedures for grading in school. This awareness occurs to varying degrees in the different schools. It seems that some schools operationalize validity in assessment as a singular event, using general checklists and other similar tools. Ensuring validity in assessment from the classroom to the national level is not a uniform procedure where teachers apply the general checklists or control tools. Validity is rather a pervasive aspect of all assessment processes that must be ensured through awareness and procedures for continuous and critical evaluation. A more formal theoretical understanding of the validity of assessment may contribute to the professionalization of educators in this field.