Currently this is the book providing a thorough introduction and a unified theoretical basis for the interpretation of equilibrium transport processes in amorphous hydrogenated tetrahydrally coordinated semiconductors - a topic of great interest to physicists and material scientists (first devices for practical applications are already being manufactured). Most of the relevant literature is reviewed with particular emphasis on the approach developed by the authors. It explains most of the experimental data and allows the extraction of information about microscopic transport processes and parameters from equilibrium transport data. This work treats electronic transport in the mentioned type of semiconductors and in particular in a-Si:H and a-Ge:H. From elementary concepts the theory is developed towards higher degrees of completeness and sophistication. Further refinements for coping with the complexity of real systems are given. The comparison of theory with experiment is an important part of the book.