Book excerpt: The 20th Century has been called the Century of Physics. It could be even more appropriate to call it the Century of Solid State Physics. All the technological developments which had changed the world by the end of the century had been based upon previous scientific developments in Solid State Physics. The Braggs, Debye, Bardeen, Landau were certainly at the forefront of all those revolutionary changes. Readership: Final-year undergraduates, graduate students, teachers, researchers working in materials physics, condensed matter/solid-state physics.