Social and behavioral researchers often draw on archival data – data collected by an entity other than the research team – to conduct scientific inquiry. Researchers typically seek to make claims about measured variables that extend beyond the measures themselves, such as interpreting a measure as representing an unobservable theoretical construct. Though researchers using archival data encounter many issues, this chapter focuses on two that have received less attention. The first concerns how researchers should justify the interpretations and uses they attach to archival measures. The second concerns how to justify generalizing findings. This chapter provides a framework to help researchers address these issues by drawing on contemporary validity theory in education and psychology as well as theory regarding causal mechanisms from philosophy and sociology. These concepts are illustrated using multiple examples from published studies.