The In Nomine, a practice of composing textless polyphonic works around a cantus firmus derived from an excerpt of a mass by John Taverner, emerged as a unique tradition in mid-sixteenth century England and immediately flourished among composers both famous (Byrd and Tallis) and long forgotten (Strogers and Woodcock). Despite the In Nomine’s recent invention, it was of singular interest to composers and became one of the most popular cantus firmus forms in Elizabethan England. Previous scholarship has pointed to the technical experiments and derivative imitation that characterize many In Nomines as signs of compositional immaturity and ascribed them to the youthful efforts of composers-in-training, positing that the In Nomine’s popularity resulted mainly from its utility in the training of choristers in composition and viol playing. My work reconsiders these experimental and intertextual aspects of the sixteenth-century In Nomine as evidence that Elizabethan composers cultivated the In Nomine as a means of asserting their belonging within a musical community. It was music for musicians—a single, flexible template through which composers could show off, reference prior works and other traditions, and create in-jokes with each other. These pieces contain many seeming contradictions: they are experimental yet also full of pre-existing material; they are non-liturgical, yet their Sarum cantus firmus is potentially fraught with religious and political significance; they have antiquarian features as well as innovative ones; and they are both compositional and performative. In chapter one, I use the concept of musical mobility to trace the musical networks through which the In Nomine, manuscripts, and instruments flowed. Chapters two and three construct an analytic framework for understanding the sophistication and breadth with which In Nomine composers drew from different musical traditions: chapter two considers the influence of continental genres and instrumental techniques while chapter three looks at the influence of speculative music and rhythmic complexity. Chapter four reexamines Elizabethan practices of citation and emulation through the lens of intertextuality, synthesizing the analytical insights of the previous two chapters and showing how the mobile musical networks explored in chapter one are instantiated through and among the extant musical traces of the sixteenth-century In Nomine.