This dissertation explores the earliest extant works of music theory printed in England, beginning in the 1590s and continuing into the 1630s. I focus on commercially available writings on music that were sold to a growing group of middle- and upper-class customers, placing the well-known writings of authors like Thomas Morley and Thomas Campion in dialogue with lesser-known authors like Thomas Ravenscroft or the anonymous author of The Pathway to Musicke. The goal of this project is to deeply situate these writings and their authors into the cultural, religious, and political climate from which they emerged. By doing this we gain a view of these treatises as engaged with the larger world of music and society. Through this approach, the divergent aims, genres, and intended audiences of this varied body of writing come to light. We are no longer left with a homogenous group of treatises that is uniformly practical in focus. Instead, a diversity of authorial claims and educational projects emerge.One of the major themes that I develop in this research is the contested and changing nature of musica practica in England. This category is neither straightforward nor uniform. It is also remarkably slippery, concealing as much as it reveals. If these treatises are focused on practical music (as they are often taken to be), it is only in a qualified and highly context-dependent sense. For many of the authors studied here, the category of practical music contains elements that have little to do with the actual practice of writing or performing music and more to do with other projects such as professional self-fashioning. By focusing on the individuality of these writings, we gain a more complete and more complicated picture of the work of music theory in early modern England.