Aleksander Gieysztor was a Polish medievalist historian. He was born in Moscow to a Polish family. He lived in Warsaw since the age of five. He graduated in history from Warsaw University in 1937. He was married to Irena Gieysztorowa, a fellow historian. The Aleksander Gieysztor Prize of the Kronenberg Foundation and the Aleksander Gieysztor Academy of Humanities are named after him.

Ostrogorsky was born at Saint Petersburg, Russia, the son of a secondary school principal and a writer on pedagogical subjects. He completed his secondary education in a St. Petersburg classical gymnasium and thus acquired knowledge of Greek early in life. He began his university studies in Heidelberg, Germany (1921), where he devoted himself initially to philosophy, economics, and sociology, though he also took classes in classical archeology. His teachers included Karl Jaspers, Heinrich Rickert, Alfred Weber and Ludwig Curtius. His interest in history, especially Byzantine history, was awakened by a young Dozent by the name of Percy Ernst Schramm. After studying various aspects of Byzantinology in Paris (1924–1925), Ostrogorsky received his doctorate from the University of Heidelberg (1925) with a dissertation in which he combined his interests in economics and Byzantine history. He then taught as Privatdozent in Breslau from 1928 and moved to Belgrade in 1933. Ostrogorsky taught at the University of Belgrade's Faculty of Philosophy, where he was the Chair for Byzantinology. Ostrogorsky made the Kingdom of Yugoslavia his permanent home and taught at Belgrade for 40 years until his retirement in 1973, leaving the Chair for Byzantinology to Božidar Ferjančić. He was made a corresponding Member of the Serbian Academy of Sciences and Arts in 1946 and a Regular Member two years later. An Institute of Byzantinology was created within the Academy in 1948 with himself as director, a post he held until his death. He was chief editor of the Institute's house organ, the Zbornik Radova Vizantološkog Instituta, through its 16th volume which appeared in 1975. He also supervised the monograph series of the Institute of which the choice items were his own study Pronija (1951) and the multivolume collection of Byzantine Sources for the History of the Nations of Yugoslavia. Ostrogorsky repaid in more than one way the hospitality he met with in his new country; he created a new generation of Yugoslav Byzantinists, broadened the horizons of Yugoslav historians by the example of his personal research, and provided for them closer contacts with the world scholarly community. Under his guidance the Belgrade Institute became, along with Munich, Paris, and Dumbarton Oaks, a leading center of research in the field of Byzantinology. Ostrogorsky remained faithful to Belgrade to the very end, although over the years suggestions were made that he take up residence in an American or Soviet center of Byzantine studies. His best known work was the standard History of the Byzantine State (German: Geschichte des byzantinischen Staates), a work which saw 3 German editions (1940, 1952, 1963) and 2 English editions (1957, 1968) and translations into more than 10 other languages. Ostrogorsky died at Belgrade in 1976.