Mark Twain Project Online applies innovative technology to more than four decades' worth of archival research by expert editors at the Mark Twain Project. It offers unfettered, intuitive access to reliable texts, accurate and exhaustive notes, and the most recently discovered letters and documents.

Its ultimate purpose is to produce a digital critical edition, fully annotated, of everything Mark Twain wrote. MTPO is produced by the Mark Twain Papers and Project of The Bancroft Library in collaboration with the University of California Press; the site is hosted by UC Berkeley's Library Systems Office. During 2005–8 the California Digital Library collaborated in MTPO's creation and initial development.