[1] He worked for fourteen years at World Bank beginning in 1987 until 2001 where he assumed various positions including that of Lead Economist.

Following his tenure at the World Bank he became Professor of International Finance Policy at the University of Amsterdam where he remained for three years and still is on the faculty.

During his time at Wharton he was a Research Fellow and taught International Financial Management, Micro-economic Theory & Monetary Economics to aspiring MBA students.

He left the Bank in January 2007 to join The International Monetary Fund IMF as the Chief of the Financial Studies Division in the Research Department.

[citation needed] Claessens has worked on various countries, including operational missions to: Argentina, Belarus, Brazil, Bulgaria, China, Chile, Colombia, Costa Rica, Czech Republic, Hungary, India, Indonesia, South Korea, Kyrgyz Republic, Malaysia, Mexico, Nigeria, Papua New Guinea, Poland, the Philippines, Romania, Russia, Slovakia, South Africa, Thailand, and Turkey.