This book presents a wide panorama of methods to investigate the spectral properties of block operator matrices. Particular emphasis is placed on classes of block operator matrices to which standard operator theoretical methods do not readily apply: non-self-adjoint block operator matrices, block operator matrices with unbounded entries, non-semibounded block operator matrices, and classes of block operator matrices arising in mathematical physics.The main topics include: localization of the spectrum by means of new concepts of numerical range; investigation of the essential spectrum; variational principles and eigenvalue estimates; block diagonalization and invariant subspaces; solutions of algebraic Riccati equations; applications to spectral problems from magnetohydrodynamics, fluid mechanics, and quantum mechanics.Contents:Bounded Block Operator Matrices:The Quadratic Numerical RangeSpecial Classes of Block Operator MatricesSpectral InclusionEstimates of the ResolventCorners of the Quadratic Numerical RangeSchur Complements and Their FactorizationBlock DiagonalizationSpectral Supporting SubspacesVariational Principles for Eigenvalues in GapsJ-Self-Adjoint Block Operator MatricesThe Block Numerical RangeNumerical Ranges of Operator PolynomialsGershgorin's Theorem for Block Operator MatricesUnbounded Block Operator Matrices:Relative Boundedness and Relative CompactnessClosedness and Closability of Block Operator MatricesSpectrum and ResolventThe Essential SpectrumSpectral InclusionSymmetric and J-Symmetric Block Operator MatricesDichotomous Block Operator Matrices and Riccati EquationsBlock Diagonalization and Half Range CompletenessUniqueness Results for Solutions of Riccati EquationsVariational PrinciplesEigenvalue EstimatesApplications in Mathematical Physics:Upper Dominant Block Operator Matrices in MagnetohydrodynamicsDiagonally Dominant Block Operator Matrices in Fluid MechanicsOff-Diagonally Dominant Block Operator Matrices in Quantum MechanicsReadership: Mathematicians, physicists and engineers.