Aad Kleijweg studied mathematics at the University of Leiden.
From 1981 until 1997 he worked at the Economic Institute for Small and Medium-sized Business (EIM). His latest function at EIM was account manager competition research. At EIM he wrote many articles and reports, the most recent years especially in the area of Small Business Economics, Competition and Market Dynamics.
From August 1997 to present, he has been working at the Netherlands Competition Authority. He started as an expert at the Antitrust Department (1997 - 2000) and was successively head of the unit Communications, Technology, Media and Transport at the Antitrust Department (2001 - 2005) and deputy and acting director of the Merger Control Department (2006 - 1 June 2008). Since 1 June 2008 he has been Deputy Director of the integrated Antitrust and Merger Control Department.