Constitutional law
The handbook "Constitutioneel Recht" (Constitutional Law, for which Kortmann received the Thorkbecke Prize in 1996), "Het staatsrecht van de landen van de Europese Unie" (Constitutional law of the Member States of the European Union), and "Het bestuursrecht van de landen der Europese Gemeenschappen" (Public Law of the Member States of the European Communities) are amongst his most important publications. The latter two books were edited by Kortmann in cooperation with professor Lucas Prakke.