Digital critical editions hold the promise of supporting new scholarly research activities not previously possible or practical with print critical editions. This promise resides in the specific ability to integrate corpora, their associated editorial material and other related content into system architectures and data structures that exploit the strengths of the digital publishing environment. The challenge is to do more than simply create an online copy of the print publication, but rather to provide the kind of resource that both eases and extends the research activities of scholars. Authoritative collections published online in this manner, and with the same rigor brought to the print publishing process, offer scholars: the ability to discover more elusive, granular pieces of information with greater facility; tighter, more obvious and more accessible connections between authoritative versions of texts, editorial matter and primary source material; and continually corrected and expanded “editions,” no longer dependent upon the print lifecycle. This paper will explore these benefits and others as they are instantiated in the recently released Mark Twain Papers Online (MTPO) (, created and published as a joint project of the Mark Twain Papers & Project at The Bancroft Library of UC Berkeley (the Papers), the University of California Press (UC Press), and the California Digital Library of the University of California (CDL). This current release of MTPO is comprised of more than twenty three hundred letters written between 1853 and 1880; over twenty eight thousand records of other letters with text not held by the Papers; nearly one hundred facsimiles; and makes available the many decades of archival research on the part of the editors at the Papers. Of particular focus in this discussion will be several key features of the system which, despite the many challenges they presented in development, were felt to be essential pieces of a digital publication that could support scholarship in new and significant ways. Those features include facets, which create intellectual structure and support serendipity; advanced search, which provides a means for researchers to apply their own analytical frameworks; citation support functionality, which serves to secure and record the outcomes of research exploration; and complex displays of individual letters, which allow detailed inspection by collocating the pieces of the authoritative object. These features together maintain the integrity and stability of the collection, while concurrently allowing for fluidity in the continued expansion of the material. In this way, MTPO hopes to succeed as a digital critical edition that will support and extend the research activities of scholars.