Semiconductor science and technology is the art of defect engineering. The theoretical modeling of defects has improved dramatically over the past decade. These tools are now applied to a wide range of materials issues: quantum dots, buckyballs, spintronics, interfaces, amorphous systems, and many others. This volume presents a coherent and detailed description of the field, and brings together leaders in theoretical research. Today's state-of-the-art as well as tomorrow’s tools are discussed: the supercell-pseudopotential method, the GW formalism,Quantum Monte Carlo, learn-on-the-fly molecular dynamics, finite-temperature treatments, etc. A wealth of applications are included, from point defects to wafer bonding or the propagation of dislocation. TOC:1. Defect Theroy: An Armchair History.- 2. Supercell Methods for Defect Calculations.- 3. Marker-Method Calculations for Electrical Levels Using Gaussian-orbital Basis-sets.- 4. Dynamical Matrices and Free Energies.- 5. The Calculation of Free Energies in Semiconductors: Defects, Transitions and Phase Diagrams.- 6. Quantum Monte Carlo Techniques and Defects in Semiconductors.- 7. Quasiparticle Calculations for Point Defects at Semiconductor Surfaces.- 8. Multiscale Modelling of Defects in Semiconductors: A Novel Molecular Dynamics Scheme.- 9. Empirical Molecular Dynamics: Possibilities, Requirements, and Limitations.- 10. Defects in Amorphous Semiconductors: Amorphous Silicon.- 11. Light-induced Effects in Amorphous and Glassy Solids.