Chaplains are America''s hidden religious leaders. Required in the military, federal prisons, and Veterans Administration Medical Centers, chaplains also work in two-thirds of hospitals, most hospices, many institutions of higher education, and a growing range of other settings. The chaplains of the U.S. House and Senate regularly engage with national leaders through public prayer and private conversation. Chaplains have been present at national protests, including the racial justice protests that took place across the country in 2020. A national survey conducted in the United States in 2019 found that 21% of the Americans public had contact with a chaplain in the prior two years. Contact with chaplains likely increased with the COVID-19 pandemic, which thrust chaplains into the spotlight, as they cared for patients, family members, and exhausted and traumatized medical staff fighting the pandemic in real time. Wendy Cadge steps back to ask who chaplains are, what they do across the United States, how that work is connected to the settings where they do it, and how they have responded to and helped to shape contemporary shifts in the American religious landscape. She focuses on Boston as a case study to show how chaplains have been, and remain, an important part of institutional religious ecologies, both locally and nationally. She has combed through the archives of major Boston institutions including the city government, police and fire department, hospitals, universities, rest and rehabilitation centers, the Catholic church, and several Protestant denominations, as well as the Boston Globe, to chart the work of chaplains historically. Cadge also interviewed over one hundred chaplains who work in greater Boston and shadowed them whenever possible, going on board container ships, walking through homeless shelters, and attending religious services at local prisons. The result is a rich study of a little-noticed but essential group of religious leaders.