Nanostructured hybrid organic–inorganic solids are a class of materials of growing interest because of their great potential in synthesis and applications. All the molecular organic precursors containing at least two Si(OR)₃ groups can be transformed into silica-based hybrid materials by hydrolytic sol–gel polycondensation. These materials exhibit the chemical and physical properties of the organic units included in the silica matrix. They present both nanometric and micrometric scale organization in the solid state, whatever the nature and the geometry of the organic units (linear, twisted, planar and even tetrahedral). These materials are prepared under kinetic control with a corresponding organization that is completely different to crystalline order. Self-organization on a nanometric scale occurs in solution due to the irreversible Si–O–Si bond formation that brings the organic units close to each other, favoring supramolecular van der Waals type interactions by decreasing the entropy. Micrometric scale organization takes place during ageing in the solid state. Densification of the Si–O–Si framework and reorganization of the micrometric aggregates generates stresses that are released by the formation of cracks that lead, upon propagation, to a birefringence phenomenon.