section transitions within which crystalline solids endure structural alterations current an engaging challenge within the interaction among the crystal constitution and the ordering technique. this article, meant for readers with a few earlier wisdom of condensed-matter physics, emphasizes the fundamental physics in the back of such spontaneous structural adjustments in crystals. beginning with the appropriate thermodynamic rules, the booklet discusses the character of order variables and their collective movement in a crystal lattice; in a structural section transition a singularity in this type of collective mode is chargeable for the lattice instability, as published by way of tender phonons. This mechanism is similar to the interaction of a charge-density wave and a periodically deformed lattice in low-dimensional conductors. The textual content additionally describes experimental tools for modulated crystal constructions and offers examples of structural adjustments in consultant structures. The booklet is split into components. the 1st, theoretical, half comprises such issues as: the Landau thought of section transitions; information, correlations and the mean-field approximation; pseudospins and their collective modes; gentle lattice modes and pseudospin condensates; lattice imperfections and their function within the part transitions of actual crystals. the second one half discusses experimental reviews of modulated crystals utilizing x-ray diffraction, neutron inelastic scattering, mild scattering, dielectric measurements, and magnetic resonance spectroscopy.