Catholic sisters and nuns in the United States have played a major role in American religion, education, nursing and social work since the early 19th century. In Catholic Europe, convents were heavily endowed over the centuries, and were sponsored by the aristocracy. Religious orders were founded by entrepreneurial women who saw a need and an opportunity, and were staffed by devout women from poor families. The number of Catholic nuns grew exponentially from about 900 in the year 1840, to a maximum of nearly 200,000 in 1965, falling to 56,000 in 2010. According to an article posted on CatholicPhilly.com, the website of the Archdiocese of Philadelphia, Pennsylvania, in October 2018, National Religious Retirement Office statistics showed that number as 47,160 in 2016, adding that “about 77 percent of women religious are older than 70.” In March 2022, the NRRO was reporting statistics from 2018, citing the number of professed sisters as 45,100. The network of Catholic institutions provided high status lifetime careers as nuns in parochial schools, hospitals, and orphanages. They were part of an international Catholic network, with considerable movement back and forth from Britain, France, Germany and Canada.

A religious sister in the Catholic Church is a woman who has taken public vows in a religious institute dedicated to apostolic works, as distinguished from a nun who lives a cloistered monastic life dedicated to prayer and labor, or a canoness regular, who provides a service to the world, either teaching or nursing, within the confines of the monastery. Nuns, religious sisters and canonesses all use the term "Sister" as a form of address.