This volume contains papers written by participants of the 6th Workshop on - erator Theory in Krein Spaces and Operator Polynomials, which was held at the Technische Universit. at Berlin, Germany, December 14 to 17, 2006. This workshop was attended by 67 participants from 14 countries. The lectures covered topics from spectral and perturbation theory of linear operators in inner product spaces and from operator polynomials. They included the theory of generalized Nevanlinna and Schur functions, di?erential operators, singular perturbations, de Branges spaces, scattering problems, block numerical ranges, nonnegative matrices and relations. All these topics are re?ected in the present volume. Besides, it contains an after dinner speech from an earlier wo- shop, which we think may be of interest for the reader, as well as a speech on the occasion of the retirement of Peter Jonas. It is a pleasure to acknowledge the substantial ?nancial support received from the - Deutsche Forschungsgemeinschaft (DFG), - Berlin Mathematical School (BMS), - DFG-Forschungszentrum MATHEON "Mathematik fur .. Schlussel- .. technologien", - Institute of Mathematics of the Technische Universit. at Berlin. WewouldalsoliketothankPetraGrimbergerforhergreathelpintheorganisation. Without her assistance the workshop might not have taken place.