The German word Zwischenfach often refers to opera roles and singers whose voices lie between the categories of mezzo-soprano and soprano. While the term is not universally accepted as a voice category, Zwischenfach voices and roles are being discussed more openly and with more specificity in collegiate and professional circles. This document includes a discussion on the challenges of categorizing dramatic voices, mezzo-soprano voices, and those who could be considered Zwischenfach, taking into consideration the inherent ambiguity and flexibility within these voice categories. The elements that have led to developmental changes in opera voices and their categories over the centuries provide insight and context on how Zwischenfach has become a term that describes the ambiguity and challenge of classifying opera voices in the twenty-first century. A main focus of this document is a discussion of eleven pieces from twenty-first century American operas which a Zwischenfach singer could consider for auditions and performances. Operas included are: Dead Man Walking by Jake Heggie, The Grapes of Wrath by Ricky Ian Gordon, After Life and Glory Denied by Tom Cipullo, Lysistrata by Mark Adamo, Dinner at Eight by William Bolcom, and Fantastic Mr. Fox by Tobias Picker. My hope is that this document will give Zwischenfach voices a resource when choosing twenty-first century repertoire and promote further discussion and acceptance of the Zwischenfach voice category.