Stigmatization is a deeply contextual, dynamic social process; stigma from abortion is the discrediting of individuals as a result of their association with abortion. Abortion stigma is under-researched and under-theorized, and the few existing studies focus only on women who have had abortions. We build on this work, drawing from the social science literature to describe three groups whom we posit are affected by abortion stigma: Women who have had abortions, individuals who work in facilities that provide abortion, and supporters of women who have had abortions, including partners, family, and friends, as well as abortion researchers and advocates. Although these groups are not homogeneous, some common experiences within the groups--and differences between the groups--help to illuminate how people manage abortion stigma and begin to reveal the roots of this stigma itself. We discuss five reasons why abortion is stigmatized, beginning with the rationale identified by Kumar, Hessini, and Mitchell: The violation of female ideals of sexuality and motherhood. We then suggest additional causes of abortion stigma, including attributing personhood to the fetus, legal restrictions, the idea that abortion is dirty or unhealthy, and the use of stigma as a tool for anti-abortion efforts. Although not exhaustive, these causes of abortion stigma illustrate how it is made manifest for affected groups. Understanding abortion stigma will inform strategies to reduce it, which has direct implications for improving access to care and better health for those whom stigma affects.