This book covers the absorption of laser energy in semiconductors and its relaxation via different sub-systems (electron-hole plasma, lattice excitations and strain, point defects, and surface electromagnetic waves) and how these lead to a threshold occurrence of surface instabilities with formation of ordered transient and permanent structures (for example, periodic surface relief, defect-deformational structures, and structural phase transitions). Formation of such structures leads to transient and permanent structural and morphological modifications of the surface of semiconductors, accompanied by changes of their basic properties. This book is a comprehensive survey of the theoretical approaches developed by the authors in the field of laser-induced surface instabilities. These approaches are compared with available experimental results.