In 1902 he earned his habilitation with the work "Miha Madijev de Barbezanis" and became the private assistant professor for Croatian history from the 12th to the 15th century (i.e. until 1409).
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.
[5] Other notable Šišić's books are the important monograph on Hrvoje Vukčić Hrvatinić and a critical edition of Dukljanin's Chronicles.
His opus, however, is the high point of "genetic history" or the archivist's approach, which patiently weaves the tapestry of the chosen period without engaging in speculations that are sometimes necessary to fill the gaps left open by archive materials.
Also, Croatian historiography came to be dominated by the multidisciplinary approach combining demographics, culturology, history of economy and art only in the second half of the 20th century, putting Šišić in the position of a classic who laid inestimable foundations but cannot be the role model for contemporary historical science.
[5] Another relevant aspect of his work is the personal library he built during his life, containing more than 20,000 titles (books, articles, historiography collections).