Measuring childlessness is complicated by the increasing complexity of family structure. Using data from the 2014 Health and Retirement Study, in this research note we compared three definitions of childlessness: (1) respondent never fathered/gave birth to a child, (2) respondent had no children who were living and in contact, and (3) respondent and spouse/partner had no children or stepchildren who were living and in contact. Results showed that the prevalence of childlessness among Americans aged 55 or older ranged from 9.2% to 13.6% depending on which definition was used. The association between select individual characteristics (gender and marital status) and the likelihood of childlessness, as well as the association between childlessness and loneliness and living arrangements, also varied depending on how childlessness was defined. Therefore, how we define childlessness can affect our understanding of its prevalence, correlates, and relationships with well-being. Future research on childlessness should carefully consider the choice of definition and its implications for research and policy discussions.