Book excerpt: These notes essentially reproduce lectures given at the International School for Advances Studies (Trieste) and at the Scuola Normale Superiore (Pisa) on various occasions. The scope of the short series of lectures was to extend on general grounds, also to mathematicians, the phenomenon of Spontaneous Symmetry Breaking (SSB), a mechanism which seems at the basis of the recent developments in theoretical physics (from statistical mechanics to many-body theory and to elementary particle theory). Besides its extraordinary success, the idea of SSB deserves being discussed also because of its innovative philosophical content and in our opinion it should be part of the background knowledge for mathematical and theoretical physics students, especially those who are interested in questions of principle and in general mathematical structures.