Validity in the field of educational assessment refers to the strength of the relationship between what you intend to measure and what you have actually measured. Valid interpretations of scores are accurate, meaningful, and useful. In this chapter, the authors draw from psychometric literature to define a set of principles for achieving validity in classroom assessment. They suggest that an assessment is valid when it is aligned with standards, objectives, and instruction. Assessments should elicit the thinking processes that are most relevant to the construct that is measured (for instance, students should think computationally when solving math problems). Assessment scoring and interpretation should be free of bias, and assessment interpretations should be based on evidence from multiple sources. Finally, tests and assessments should be useful for their purpose, whether it be identifying gaps in student understanding or demonstrating student achievement on a relevant standard. The authors provide guidelines for teachers in designing, implementing, scoring, and interpreting assessment, and they suggest teachers emphasize validity in all aspects of their assessment.