In this volume we present a collection of research papers which mainly follow lectures given at the “Colloquium on Operator Theory”. This conference was held at the Vienna University of Technology in March 2004 on the occasion of the retirement of Heinz Langer. The present volume of the series “Operator Theory:
Advances and Applications” is dedicated to him and his scientific work. The book starts with an introductory part which provides some information about the colloquium itself. We have also included the laudation given by Aad Dijksma and a list of the recent publications of Heinz Langer, which updates his
bibliography given in OT 106. The main part of the book consists of fifteen original research papers, which deal with various aspects of operator theory and indefinite inner product spaces. It concludes with a historical survey on the theory of positive definite functions.