The monograph "Upper Cretaceous Calcareous Nannoplankton Biostratigraphy in Bulgaria" provides a comprehensive analysis of the biostratigraphic distribution of calcareous nannoplankton from the Upper Cretaceous period in Bulgaria and the Black Sea. It is based on a variety of geological sections representing different facies types, including epicontinental chalk, carbonate sequences, and transitional deposits. The work presents a new nannofossil zonation that includes 19 established zones and 15 newly defined subzones, based on reliable nannofossil events. This zonation deviates from the commonly used standard zonations as it is specifically tailored to the geological conditions of Bulgaria. The monograph summarizes the nannofossil data from representative sections and key outcrops, providing a reliable regional framework that correlates with the globally recognized standard zonations of the Upper Cretaceous period.