The book describes the basic principles that relate to field and current inhomogeneities in semiconductors and their kinetics that occur in the regime of negative differential conductances of semiconductors. The book presents the related theory and experiment. It proceeds to give for the first time the experimental methods to observe directly these inhomogeneities within the semiconductor. It analyses in detail the different ranges in which such inhomogeneities occur, when they are stationary and when not and what technical and device application result. The accompanying film on the website demonstrates all related kinetic effects. Information on these effects was previously mostly available indirectly by interpretation of current-voltage characteristics, or by point contact probing along the surface, or by changes in the luminescence spectrum. The material is based on the original papers of the research team of the author, starting in the late 50s and updated to incl. 2008.