The Counter Reformation caused the most dramatic musical shift the Catholic Church had seen in centuries. One of the key actors in this change were the Jesuits. Their expansion resulted in the dissemination of new music across the globe, reaching China, the Philippines and Mexico. But despite substantive research on Jesuit music elsewhere, we know little about their musical legacy in the Low Countries. Understanding this facet of Jesuit music is nevertheless crucial, since their position in the United Provinces and Spanish Netherlands was unique. Unlike in the Spanish Empire, multiple factors in the Low Countries tempered the Jesuits’ influence, particularly during the Eighty Years’ War. During this period of instability, the Jesuits endured persecution from both Dutch Calvinists and fellow Catholics such as the Jansenists: their institutions were closed, their activities limited, and many of their documents lost. Despite a lack of key sources, however, many Jesuit documents survive in private archives. My goal is to examine these undiscovered sources, and to grasp how the Jesuits reformed Low Countries music in this unique era. I will create the first survey of Jesuit-printed music books in the Low Countries. Having examined these neglected documents, I will offer the first full appraisal of the Jesuits’ role in the music industry in the region. Such a project will offer a better and fairer picture of the Jesuits’ musical activities at large, both in Europe and globally.