Content validation theory and practice have received considerable attention in the nursing research literature. This paper positions the discourse within the broader scientific literature on validity of measurement. The content validity index has been recommended as a means to quantify content validity; this paper critically examines its origins, theoretical interpretations, and statistical properties. In addition, the author sets out to understand why many nurse researchers are occupied with content validity and its estimation. This investigation may be of interest to the scholar who desires to deeply understand the issues surrounding validity of measurement.