Arriving at CERN in1953 to work on the design and construction of the CERN Proton-Synchrotron, he was appointed Director of PS Division in 1954 until May 1960. With the sudden death of Cornelis Bakker, he served as Acting Direct or-General (May 1960 - July 1961) until Victor Weisskopf was appointed the following year.

A member of the Committee for Scientific Policy (1965-68) he was never fully absent from CERN during the 1960s.

In June 1969, CERN Council appointed him Director of the 300 GeV Project, and Director-General designate for the new 300 GeV Laboratory, which was to become CERN Laboratory II.

In February 1971, following the decision to continue with the eight year programme, he was appointed Director-General for CERN Laboratory II, with responsibility for the design and construction of the SPS accelerator. Willibald Jentschke became Director-General of CERN Laboratory I (Meyrin, Switzerland).

In 1975 CERN Lab I and Lab II were fused in one Laboratory CERN. From 1976 - 1980 he served as Executive Director-General, with Léon Van Hove as Director-General for Research.

His links with CERN continued after his term of office as Executive Director-General, as Chairman of the International Committee for Future Accelerators, as well as serving on committees and panels within the European Communities.