The liturgical reforms of Vatican II have issued in two signal developments in modern American Catholic worship: the rise of more fully active participation by the lay faithful, and the virtual disappearance of the historic body of western church music, Gregorian chant. These two profound developments, however, were not a sudden product of Sacrosanctum concilium, but rather had a long, turbulent, and intertwined history in the decades of the twentieth century leading up to the council. That history began with the promulgation of the motu proprio Tra le sollecitudini in 1903 by Pope Pius X, which tied Gregorian chant and the active participation of the laity together. This dissertation seeks to document the history of how Tra le sollecitudini "played out" in the United States, looking at its reception by clergy, musicans, and lay worshipers. The discussion is grounded in an overview of lay liturgical singing back to the early church, the history of papal legislation on music, and the debate as it unfolded in the twentieth century (charted particularly through the major Catholic music journals.) In particular the notion of "full, conscious, and active participation" is shown to have had the sponsorship of all the pontiffs of the twentieth century back to Pius X, and the significant efforts to implement that participation through music and especially chant are chronicled, down to their end in frustration.  In our day of liturgical retrenchment, this research adds a cautionary note to romanticized notions of pre-Vatican II music; it shores up the commitment to fully active liturgical participation both by historical precedent and the long theological grounding of the twentieth century; and it offers a (non-political) groundwork for re-appropriating the "inestimable treasure" of the Gregorian inheritance.