This chapter examines the issue of establishing viability for de facto states. It looks into what it means for a de facto state to be viable; what the interplay is between viability on the one hand, and the need to secure de facto secession, de facto statehood, and internal legitimacy on the other; and how viability is fundamentally linked to the ability of de facto states to establish relationships with the wider world. The latter point, especially, is illustrated by unpacking cases from the Caucasus (Abkhazia and South Ossetia) and the Horn of Africa (Somaliland).