This paper provides a framework for understanding Twitter as a historical source. We address digital humanities scholars to enable the transfer of concepts from traditional source criticism to new media formats, and to encourage the preservation of Twitter as a cultural artefact. Twitter has established itself as a key social media platform which plays an important role in public, real-time conversation. Twitter is also unique as its content is being archived by a public institution (the Library of Congress). In this paper we will show that we still have to assume that much of the contextual information beyond the pure tweet texts is already lost, and propose additional objectives for preservation.