Our book presents the way Bessarabia, Bukovina and Transylvania, the historical provinces united with Romania in 1918, were integrated in the Romanian administrative structures. The book consists of three chapters, one for each of the three provinces, as follows: Chapter I. Bessarabia, Chapter II. Bukovina, and Chapter III, dedicated to Transylvania. The process of reintegrating these provinces into the structures of the Romanian administration was a long and difficult one. For each of them, the administrative autonomy was initially recognized and previous administrative structures, namely the Russian, Austrian and Hungarian ones, were maintained for a while. Gradually, elements of the Romanian central administration were transfered from Bucharest to these provinces, while still considering their specificity and traditions. Although each province registered particular stages, the process of administrative unification was a similar one. The Constitution of 1923 played an importantrole in this unification and the Law of administrative unification of 1925 completed the process.