In this article we analyze “don't know” responses from three sources of longitudinal data: the National Longitudinal Study of Adolescent Health (n = 14,528), the National Survey of Families and Households (n = 5,488), and the National Health Interview Survey Second Longitudinal Study of Aging (n = 1,131). We asked whether these responses are meaningful in family research, and, if so, how evaluating these responses can contribute to the development of theory, the discovery of novel findings, and identification of sensible methods for analyzing these nebulous responses. We found that “don't know” responses to questions about family members predicted less educational attainment, poor marital quality, and earlier mortality. Results suggest that this response category may have substantive meanings rather than indicating neutral responses or being missing data.