Most discipline-based education researchers (DBERs) were formally trained in the methods of scientific disciplines such as biology, chemistry, and physics, rather than social science disciplines such as psychology and education. As a result, DBERs may have never taken specific courses in the social science research methodology--either quantitative or qualitative--on which their scholarship often relies so heavily. One particular aspect of (quantitative) social science research that differs markedly from disciplines such as biology and chemistry is the instrumentation used to quantify phenomena. In response, this "Research Methods" essay offers a contemporary social science perspective on test validity and the validation process. The instructional piece explores the concepts of test validity, the validation process, validity evidence, and key threats to validity. The essay also includes an in-depth example of a validity argument and validation approach for a test of student argument analysis. In addition to DBERs, this essay should benefit practitioners (e.g., lab directors, faculty members) in the development, evaluation, and/or selection of instruments for their work assessing students or evaluating pedagogical innovations.