This module is designed for educators, educational researchers, and psychometricians who would like to develop an understanding of the basic concepts of validity theory, test validation, and documenting a "validity argument." It also describes how an in-depth understanding of the purposes and uses of educational tests sets the foundation for validation. We describe the benefits and limitations of educational tests, the concept of validity and why it is important, and the types of validity evidence that should be used to support the use of a test for a particular purpose. We also discuss the need for assessment programs to provide such evidence and how it should be interpreted and documented to use educational assessments to best serve education.