Most scientific work has focused on study of electron-positron reactions, the dev. of experimental techniques, and construction and operation of large detector systems. At CERN: 1984-98, working for OPAL experiment, electron-positron storage ring LEP; since 2009, Director-General. 1998, Chair, Univ. of Hamburg, established a working group to prepare experiments at an electron-positron linear collider that quickly became one of the leading groups in this field worldwide; 2004-08, Research Director, Particle and Astroparticle Physics, DESY laboratory. Member of many scientific committees and advisory bodies. Recipient of honorary Doctor of Science degrees in several universities in Europe and in Canada, India, Thailand, Australia.