Design Science Research for Information Systems (ISDSR) has received considerable attention recently. With the growing interest in ISDSR, calls continue to establish the rigor of artifact construction. In analogy to other scientific disciplines, the scientific foundation of artifact construction has been designated as IS Design Theory (ISDT) (Gregor 2006, p. 611). Although the ISDSR community has been discussing ISDTs since the early 1990s, no consensus on the definition or the componential structure of ISDTs has been reached yet. In this short article, we give an overview of the ongoing discussion on ISDTs. First, we introduce fundamental concepts of ISDT. Second, we give an overview on seminal contributions to the field of ISDTs in chronological order. Finally, we cluster the presented ISDT contributions into ISDT schools.