The higher periodicities observed on several clean crystalline surfaces are discussed. Among the models invoked to account for their formation, we focus here on electronic mechanisms, particularly of the charge-density-wave type. A one-dimensional model is introduced to illustrate the continuity between the charge-density wave picture and the chemical bond picture. The reconstruction phase transitions of W and Mo(100) and Si(111) surfaces are discussed in some detail. We also consider their critical behaviour, and the defect structure in the ordered state.