At the end of the ninth century, Denmark comprised all areas bordering on the Kattegat, that is, the central part of southern Scandinavia. This chapter explores the origins of this conglomeration of territories and of its further history throughout the rest of the Viking Age and the early Scandinavian Middle Ages. Archaeology has become an increasingly important source for the study of the earliest political history of Denmark. New procedures have been developed, such as the excavation of large areas of settlement and improved dating techniques, notably dendrochronology. By and large there appears to have been a tendency towards political unification of Denmark throughout the Viking Age, but royal sovereignty over the entire medieval Denmark cannot be substantiated until the latest part of that period. The chapter also presents discussions on the North Sea empire, and the early medieval Danish kingdom, especially the political and social organisation of eleventh-century Denmark.