As a scientist, Šišić was primarily interested in the history of the early and high Middle Ages in Croatia. His greatest work is "History of the Croats Under Home Rule", while his most popular work is "Overview of the History of the Croatian People", which was edited by Jaroslav Šidak and widely used for almost five decades. "History of the Croats under the Arpad Kings (1102-1301)", an unfinished compilation published after his death, is generally considered below the level of his "magnum opus".