Americans of the early-twentieth century were vitally interested in prayer, as the primary literature of the period, both secular and religious, attests. My dissertation explores mainline Protestants various attempts to articulate a convincing and satisfying ethic of prayer in modern culture, both intellectually and culturally. Beginning with John Tyndall in the 1870s, modern intellectuals attacked the notion of a Cod intervening in the natural world in answer to prayer. Conservative Protestants attempted to beat the skeptics at their own game by presenting empirical evidence from the prayer lives of missionaries and others that Cod did in fact answer prayer. Others chose in varying degrees to adapt traditional ideas of prayer to modern thought by emphasizing divine immanence. In addition to intellectual doubts, many Americans in the emerging middle-class culture saw themselves as too busy to practice the devotional rigors of their ancestors. While Protestants in the Keswick holiness tradition maintained a stringent, exclusivist prayer life in the face of social changes, other Protestants demonstrated a willingness to moderate Christian devotionalism and open it up to non-Protestants. They thus adapted the traditional life of prayer to fit the needs of a modern, bustling, ideologically diverse middle-class culture.