In the first part of the course we will provide a brief introduction to the spectrum of the Laplacian on Euclidean domains and Riemannian manifolds, along with a few basic examples, and their relations with physical phenomena (waves and vibrations). Then, we will focus on some classical problems in spectral geometry, such as eigenvalue bounds and isoperimetric inequalities for the eigenvalues. The questions that we will address are the following: how does the geometry and the topology of the ambient space influence the spectrum (the eigenvalues)? On the other hand, what information can give the knowledge of the spectrum on the geometry and topology of the ambient space? We will present a few classical techniques which have been adopted throughout the years to address these questions. In the final part of the course (if time allows) we will consider some recent developments on old and new problems, and we will present some open questions (next lectures: Feb 27 and Feb 28, 10 AM, Aula Seminari).