Josip Stadler (1843-1918) is a Croatian theologian, philosopher and the first Archbishop of Vrhbosna. He took his doctor's degree in philosophy (1865) and theology (1869) in Rome at the present-day Gregoriana University. He is the author of the manual Filosofija, which is comprised of six volumes: Dijalektika (1904), Kritika ili noetika (1905), Opća metafisika ili ontologija (1907), Kosmologija (1909), Psihologija (1910) and Naravno bogoslovlje (1915). In this thesis Stadler's conception of ontology and his philosophical-theological position will be treated. In the introduction, Stadler's life and work and his philosophical-theological opus are shortly presented. In the main part of the thesis the context of Stadler's philosophical-theological-pastoral works and the very Stadler's ontology will be analysed. Stadler belongs to neoscholasticism, which emerges in the middle of 19th century and develops more profoundly by publication of the encyclicals Aeterni Patris (1879) and Pascendi dominici gregis (1907). In the thesis authors that essentially influenced Stadler's philosophical-theological position in ontology and philosophy at all (Thomas Aquinas, Aristotle, Tongiorgi etc.) and authors who Stadler criticises in his Opća metafisika ili ontologija (Kant, Hegel, Descartes etc.) are mentioned. Some of the topics of Stadler's Opća metafisika ili ontologija: metaphysics, being (biće), essence, existence, space, time, substance, accident, hypostasis, person, cause, matter, form, perfection etc. After the Stadler's ontology is analysed, the significance of Stadler's ontology and his philosophy in his own time and in our days will be considered. In the conclusion main theses of the thesis are shortly exposed and a certain type of résumé is given.