When SLA researchers design or adapt an assessment for research purposes, a key concern is to validate the interpretations of the assessment. This involves creating an argument on the meaningfulness and usefulness of the interpretations of the assessment, and its appropriateness for the populations or individuals participating in the research. This chapter describes the work that has been done in SLA research with reference to instrument validation and the extent to which SLA researchers’ practices are aligned with the possible ways to plan a validity argument. It concludes with suggestions on how to improve current practices.