This volume contains a selection of papers in modern operator theory and its applications. Most of them are directly related to lectures presented at the Inter­ national Workshop on Operator Theory and its Applications held at the University of Groningen (IWOTA-98) in Groningen, the Netherlands, from June 30-July 3, 1998. The workshop was attended by 97 mathematicians-of which 12 were PhD or postdoctoral students-from 19 countries. The program consisted of 19 plenary lectures of 40 minutes and 72 lectures of 30 minutes in 4 parallell sessions. The present volume reflects the wide range and rich variety of topics presented and discussed at the workshop. The papers deal with operator polynomials and analytic operator functions, with spectral problems of (partial) differential oper­ ators and related operator matrices, with interpolation, completion and extension problems, with commutant lifting and dilation, with Ricatti equations and real­ ization problems, with scattering theory, with problems from harmonic analysis, and with topics in the theory of reproducing kernel spaces and of spaces with an indefinite metric. All papers underwent the usual refereeing process.