It has been almost 20 years since the publication of International Society and the De Facto State by Scott Pegg in 1998, the first book-length substantive theoretical attempt to investigate the phenomenon of de facto states—secessionist entities that control territory, provide governance, receive popular support, persist over time, and seek widespread recognition of their proclaimed sovereignty and yet fail to receive it. Even though most de facto states are relatively small and fragile actors, in the intervening years the study of de facto or contested or unrecognized statehood has expanded dramatically. The de facto state literature has contributed significantly to the growing recognition that the international system is far more variegated than is commonly perceived. An initial focus on the external relations of de facto states has increasingly given way to a newer focus on their internal dynamics and domestic state-building processes and on how a lack of sovereign recognition conditions but does not prohibit their democratic, institutional, and political development. Perhaps most notably, there has been an explosion in detailed empirical research based on original data, which has greatly enriched our understanding of these entities. Alas, the subfield of de facto state studies is also characterized by recurrent problems. There has been an extensive proliferation of different terms used to describe these entities, and much fighting has erupted over precise definitions, resulting in limited scholarly progress. Fundamentally, there remains a continued failure to reach agreement on the number of these entities that exist or have existed since 1945. The nuanced and empirically rich academic literature has also largely failed to advance journalists or policymakers’ understanding of de facto states. Yet, the prospects for de facto state studies remain bright. More diverse comparative work, renewed attention to how engagement without recognition might facilitate the participation of unrecognized entities in international politics, a renewed focus on parent state strategies, and increased attention to de facto states and conflict resolution are areas deserving of greater scholarly attention.