The borders of medieval Ma wara' al-Nahr (Transoxania) are typically defined as the Amu-Darya or Jayhun River and the point where the political authority of Islam ceased by both medieval and modern writers. To the north and east, these borders are imprecise and entirely dependent on changing political conditions as the authority of different Muslim regimes, most importantly the Samanids for the purposes of this paper, expanded and contracted into the Central Asian steppes. Yet, neither the imprecision nor the impermanence of these borders stopped people from trying to define the physical limits of Muslim Ma wara' al-Nahr. This paper examines the ways medieval Muslim sources attempted to define the limits of Ma wara' al-Nahr in the 3rd-4th/9th-10th centuries and the role these limits, as defined in our sources, played in medieval conceptualizations of Ma wara' al-Nahr as a frontier. Through defining the borders of Ma wara' al-Nahr, writers have also, indirectly, defined the borders of neighboring regions, most notably the Bilad al-Turk. In many cases, such definitions have created a false dichotomy between the lands of Islam and its neighbors and obscured the overlapping and intertwined nature of the eastern frontier. By connecting the borders of Ma wara' al-Nahr with the limits of Muslim political authority and then defining these borders at certain fixed points, a complex frontier zone has been reduced to a political and cultural duality which overlooks the presence of non-Muslim political authorities within what is nominally Muslim Ma wara' al-Nahr. This paper will further examine the way non-Muslim political authorities within Ma wara' al-Nahr are treated by our medieval sources. It will also explore the ways a frontier setting requires local authorities to negotiate complex political and cultural networks while seeking to identify and secure a clearly defined border.