This article argues that test takers are as integral to determining validity of test scores as defining target content and conditioning inferences on test use. A principled sustained attention to how students interact with assessment opportunities is essential, as is a principled sustained evaluation of evidence confirming the validity or calling into question the inferences for individual students. Three innovative assessment systems are highlighted to illustrate where and how the developers might handle diverse test taker needs and learning characteristics. ONPAR measures challenging content using multisemiotic methods and novel item types, designing items to handle multiple profiles so they are accessible for most students. Dynamic Learning Maps has built an innovative network of learning maps, and multiple pathways designed to model how diverse students acquire knowledge. To support their assessments, the National Center and State Collaborative has built an exemplary web of educator resources such as content modules and guides in order to support differentiated learning.