After 30 years of research, co-citation analysis has become the dominant method for the empirical study of the structures of scientific communication. There is a considerable variety of methods and, at the same time, a lack of methodological evaluation. This paper summarizes the present state of co-citation analysis and presents several methods of clustering references. The database used is a selection of 2,114 documents in the field of organization studies from 1986-2000. The evaluative study shows that the choice of methods has a strong impact on the results created. It also shows that the methods of cluster and factor analysis hitherto used have only a limited value in differentiating clearly between schools of scientific research - the 'invisible colleges'.