Van der Klis pioneered special mathematical analysis techniques that are now regarded as the “gold standard” within his discipline.

His studies in the years 1984-1987 clarified the principles for classifying the several hundreds of strong X-ray sources known to exist in our Galaxy.

In the 1990s, he and his research group, including Rudy Wijnands, discovered the first millisecond X-ray pulsar in a binary system, SAX J1808.4-3658, a neutron star spinning around its axis more than 400 times per second.

Since 2002 he is an elected member of the Royal Netherlands Academy of Arts and Sciences (KNAW)[2] and since 2003 also of the Koninklijke Hollandsche Maatschappij der Wetenschappen (KHMW).

Since 2005 he is Director of the Astronomical Institute Anton Pannekoek of the University of Amsterdam and Chair of the Netherlands Research School for Astronomy (NOVA).