1. Introduction.- 2. The Complex Dielectric-Constant Tensor ?ij(?,k) and Normal Waves in a Medium.- 3. The Tensor ?ij(?,k) in Crystals.- 4. Spatial Dispersion in Crystal Optics.- 5. Surface Excitons and Polaritons.- 6. Microscopic Theory. Calculation of the Tensor ?ij(?,k).- 7. Conclusion.- A.1 Crystal-Symmetry Notation.- A.2 Information from Space Group Theory.- A.2.1 Classification of the States of Mechanical Excitons.- Notation.- References.