This chapter seeks to address the basic question that poses itself to any listener: how can two performances of a given piece of Renaissance polyphony sound radically different and unlike. It reviews the reasons for this impression: what the sources and notation tell us (and what they don’t) regarding performance forces, treatment of pitch, tempi, proportions, and performance conditions and contexts.The chapter also offers an overview of the changes of fashion and practice since the onset of sound recordings of Renaissance music. Crucial to the question is the way in which our assumptions and biases inform our interpretation of this information (and other sources of knowledge on performance practice), just as they did those of previous generations. As in the rest of this book, I seek here to define the relationship between (and readers’ awareness of) the twin claims of knowledge and interpretation, the tension between which plays itself out in concrete terms in the domain of performance practice.