This dissertation asks a fundamental question: how do listeners make sense of the relationships between the constitutive units of post-tonal compositions? Hearing such constitutive units—that is, hearing in terms of musical forms—requires attending to a rich, dense web of interrelated ideas, moving beyond the harmonic relationships that are typically valued in formal analysis. The approach I take assumes that a wide variety of musical features—including rhythmic and melodic contour, pitch content, timbre, and texture—shape listeners’ expectations when they encounter post-tonal music. As a way of explaining how listeners manage this variety, I propose that formal units in post-tonal music can be thought of as possessing specific “affordances,” taking inspiration from cognitive scientist Don Norman’s work on material design. These affordances reflect the physical attributes of sound sequences as well as the listener’s past experiences and beliefs when encountering similar units, and offer a means of apprehending the formal units specific to a given composition. The result of this approach is a view of musical form in which the listener and composer mutually construct the significant formal units of a musical work through their interactions, a perspective particularly well adapted to the challenges presented by post-tonal music. My dissertation thus formulates a methodology for analyzing formal function in post-tonal musical contexts based on this listener-centered approach. Chapter 1 introduces the concept of post-tonal formal function, based on the notion of affordance. Chapter 2 takes a closer look at phrase in post-tonal music, demonstrating how the analyst can draw on parameters that the composer marks as salient in order to make determinations about phrase boundaries. Chapter 3 engages with large-scale formal structures in works by Arnold Schoenberg and Pierre Boulez. In Chapter 4, I explore the concept of post-tonal closure or “cadence” in works by Alfred Schnittke and György Ligeti. Chapter 5 deals with some aspects of phrase in the music of Luciano Berio. Finally, Chapter 6 discusses how my listener-centered model for formal analysis of post-tonal works may be applied in other contexts, expanding our understanding of what it means to hear formally.