This chapter introduces the two principal forms of polyphony practiced during the Renaissance: written and extemporized. The transition from manuscript to print culture is perhaps the most significant extramusical determinant for musical practice in the Renaissance period. Examples of manuscript and printed sources (including different kinds of written sources) are examined, and their implications for practice considered. Next, different forms of extemporized practices are introduced and described, including the surviving evidence for them; in turn, their implications for polyphony as a practice are considered. Finally, the two forms of practice are compared, and the relationship between the two. It is clear that most teachers of the time regarded them as interdependent, while viewing the relationship differently.