I Lightly Doped Semiconductors.- 1. The Structure of Isolated Impurity States.- 2. Localization of Electronic States.- 3. The Structure of the Impurity Band for Lightly Doped Semiconductors.- 4. A General Description of Hopping Conduction in Lightly Doped Semiconductors.- 5. Percolation Theory.- 6. Dependence of Hopping Conduction on the Impurity Concentration and Strain in the Crystal.- 7. Hopping Conduction in a Magnetic Field.- 8. Activation Energy for Hopping Conduction.- 9. Variable-Range Hopping Conduction.- 10. Correlation Effects on the Density of States and Hopping Conduction.- II Heavily Doped Semiconductors.- 11. Electronic States in Heavily Doped Semiconductors.- 12. The Density-of-States Tail and Interband Light Absorption.- 13. The Theory of Heavily Doped and Highly Compensated Semiconductors (HDCS).- III Computer Modelling.- 14. Modelling the Impurity Band of a Lightly Doped Semiconductor and Calculating the Electrical Conductivity.- References.