In the early 1900s a conjunction of factors led to an official attempt to impose a tighter degree of regulation and control by the Catholic Church over its music than had ever been seen before. Inevitably, this had significant effects on Catholic church music in England. Central to the process was the promulgation of Tra Le Sollectudini by Pius X on 23 November 1903. This chapter, then, examines the content, context and implications of this document. Subsequent chapters discuss its practical effects, especially as regards plainchant and Renaissance polyphony.