With his previous book, The Structure of Matter: From the Blue Sky to Liquid Crystals, Andre Guinier described various models for the atomic structure of matter. Here, the authors show how such models can be used to explain the macroscopic properties of solids. The aim of this text is to provide the non-specialist with an introduction to the solid state, and to examine the technological importance of recent advances in this field. A chapter on thermal properties is followed by a discussion of electrical properties, and the behaviour of electrons in metals, semi-conductors and superconductors. Magnetic properties are then considered, and the final chapters look at mechanical properties such as plasticity, dislocation and diffusion. For those who wish to study the subject in more depth, a detailed mathematical treatment is presented in boxed sections, which can be omitted on first reading.