In the 1800s, nuns moved west with the frontier, building hospitals and schools in immigrant communities. They provided aid during the Chicago fire, cared for orphans and prostitutes during the California Gold Rush, and brought professional nursing skills to field hospitals on both sides of the Civil War.

In the 1900s, nuns built the nation's largest private school and hospital systems, and brought the Catholic Church into the Civil Rights movement. As their numbers began to decline in the 1970s, many sisters were forced to take professional jobs as lawyers, probation workers, and hospital executives because their salaries were needed to support older nuns, many of whom lacked a pension system. Currently there are about 65,000 sisters in America, down from 204,000 in 1968. Their median age is sixty-nine.

Nuns became the nation's first cadre of independent, professional women. Some nursed, some taught, and many created and managed new charitable organizations, including large hospitals and colleges. Sweeping in its scope and insight, Sisters reveals the spiritual wealth that these women invested in America.