The purpose of this chapter is to present a narrative on quantitative measurement of students’ content knowledge outcomes as well as to share trends and opportunities at the crossroads of assessment, validity, and mathematics education research. It is important to understand the past in order to adequately build upon the literature in an effective and coherent fashion. Increased technologies in K-12 schools are likely to influence the appearance and format of assessments. Current assessment models will evolve and so will their associated validity arguments. This chapter also outlines gaps in needed student measures and viable directions for validation work with measures of students’ content knowledge. We hope that readers might take up ideas presented in this chapter as a way for conversations with colleagues in the same field and across fields, as a way to have broader impact. Through such conversations, scholars may better synergize research efforts, better define intellectual merit of their work across fields, and better influence quality of research that includes studying students’ content outcomes using quantitative measures.