The book is a collection of lecture notes and survey papers based on the mini-courses given by leading experts at the 2015 Seminaire de Mathematiques Superieures on Geometric and Computational Spectral Theory, held from June 15-26, 2015, at the Centre de Recherches Mathematiques, Universite de Montreal, Montreal, Quebec, Canada. The volume covers a broad variety of topics in spectral theory, highlighting its connections to differential geometry, mathematical physics and numerical analysis, bringing together the theoretical and computational approaches to spectral theory, and emphasizing the interplay between the two.