Validity in measurements in human and social sciences is commonly referred to as “measuring what one intends to measure,” and with a good fit of item parameters–somewhat simplified–it is considered to ensure validity when measuring latent traits inpersons. Despite new thinking and trends about validity and positioning validity in measurement theory and practice, today’s use of validity can mostly be traced back to the classical test theory (with no compensation for ordinality, no proper separation of latenttraits for persons and items, nor a defined measurement system). Consequently, when positioning models, measurement, and metrology to extend the SI, there is a need to critically review the concept of validity. A fundamental mistake is that, too often, a proper distinction is not made between the latent trait itself and the latent trait as measured. In the human and social sciences, where there are yet to be any established units, measurement-related validity should ideally not precede validity in the latent trait itself. Notably, the concept of validity has so far not been included in the International Vocabulary ofMetrology (JCGM, 2012), although validation processes (entry 2.45) have been included. This is reasonably due to the centuries of work contributing to a solid consensus about the quantities in themselves. However, given the urgent needs of society for new knowledge about the world to make well-informed decisions about measurements of latent traits, we do not have centuries to first reach consensus about measurement validity. Neither was this done with the existing SI, which has been an iterative work, defining the quantities and measurement processes. Therefore, in a time where the possibilities for new units to extend the SI are being explored, aniterative and cross-disciplinary effort isneeded. Thus, this chapter reviews and discusses validity and its related aspects. Finally, the chapter concludes with a proposed call for action to include a nuanced view of validity when extending models, measurement, and metrology of the SI to include measurement in the human and social sciences.