Interpersonal acceptance-rejection theory (IPARTheory) is an evidence-based theory of socialization and lifespan development that aims to predict and explain major consequences and other correlates of interpersonal acceptance and rejection worldwide (Rohner, 2014; Rohner and Lansford, 2017). The theory was formerly known as parental acceptance-rejection theory (PARTheory), reflecting its emphasis on the effects of perceived acceptance-rejection by parents (Rohner, 1986, 2004; Rohner and Rohner, 1980). The theory has been expanded to include significant interpersonal relationships more broadly, such as intimate relationships with friends and romantic partners (Rohner, 2014). Accordingly, PARTheory was renamed IPARTheory to reflect this shift in focus (Rohner, 2014). Despite changes in name and emphasis, significant portions of the theory continue to feature the effects, causes, and other correlates of children’s perceptions of parental acceptance-rejection, and of adults’ remembrances of parental acceptance-rejection in childhood. In this chapter, we focus specifically on parental acceptance-rejection. We note that the term “parent” refers to whomever the major caregiver(s) is/are of a child—not necessarily only to biological or adoptive parents.