This book presents a wide panorama of methods to investigate the spectral theory of bounded and unbounded block matrices of linear relations. It explains some conditions to prove some Frobenius–Schur decompositions for linear relations and characterizes the stability of the essential spectra of these linear relations. It focuses on the study of the Fredholm theory in both Banach and Hilbert spaces, the local spectral theory of multivalued linear operators and the block matrix of linear relations. As a pioneering literature discussing spectral theory of bounded and unbounded block matrices of linear relations, this book attempts to contribute to the scarce literature on the topic. It gathers the minimum needed background material which allows a relatively friendly access to the book. Detailed proofs of all theorems are exhibited with accuracy and clarity, allowing students to thoroughly familiarize themselves with all the basic concepts.