After his PhD research, he moved back to Amsterdam where he worked from 1990 to 1995 in the SWI Department under Professor Bob Wielinga, on the use of reflection in expert systems, on the formal underpinnings of the CommonKADS methodology for Knowledge-Based Systems. In 1995 he joined the AI research group at the Vrije Universiteit Amsterdam, where he co-lead the On-To-Knowledge project, one of the first Semantic Web projects. He was appointed full professor in 2002, and is leading the Knowledge Representation and Reasoning Group. Currently he is scientific director the LarKC project aiming to develop the Large Knowledge Collider, a platform for very large scale semantic web reasoning.