The Centre for Science and Technology Studies (CWTS), Leiden University, has developed a new ranking system entirely based on its own bibliometric indicators. This web-publication is the first in a series of rankings. The work focuses on all universities worldwide with more than 700 Web of Science indexed publications per year. This implies that the about 1000 largest (in terms of number of publications) universities in the world are covered, and that our bibliometric analysis is based on the scientific output of many hundreds of active researchers in each of these universities.