This volume contains a collection of recent original research papers in operator theory in Krein spaces, on generalized Nevanlinna functions, which are closely connected with this theory, and on nonlinear eigenvalue problems. Key topics: - spectral theory for normal operators in Krein spaces - perturbation theory for selfadjoint operators in Krein spaces - models for generalized Nevanlinna functions

Partial Non-stationary Perturbation Determinants for a Class of J-symmetric Operators.- Reproducing Kernel Spaces of Series of Fueter Polynomials.- Extremal Extensions of a C(?)-suboperator and Their Representations.- A Variational Principle for Linear Pencils of Forms.- Selfadjoint Extensions with Several Gaps: Finite Deficiency Indices.- The Spectrum of the Multiplication Operator Associated with a Family of Operators in a Banach Space.- A Factorization Model for the Generalized Friedrichs Extension in a Pontryagin Space.- Generalized Schur Functions and Augmented Schur Parameters.- On Nonmonic Quadratic Matrix Polynomials with Nonnegative Coefficients.- On Operator Representations of Locally Definitizable Functions.- Symmetric Relations of Finite Negativity.- An Operator-theoretic Approach to a Multiple Point Nevanlinna-Pick Problem for Generalized Carathéodory Functions.- Bounded Normal Operators in Pontryagin Spaces.- Scalar Generalized Nevanlinna Functions: Realizations with Block Operator Matrices.- Polar Decompositions of Normal Operators in Indefinite Inner Product Spaces.- Bounds for Contractive Semigroups and Second-Order Systems.