Book excerpt: Impact ionization, avalanche and breakdown phenomena form the basis of many very interesting and important semiconductor devices, such as avalanche photodiodes, avalanche transistors, suppressors, sharpening diodes (diodes with delayed breakdown), as well as IMPATT and TRAPATT diodes. In order to provide maximal speed and power, many semiconductor devices must operate under or very close to breakdown conditions. Consequently, an acquaintance with breakdown phenomena is essential for scientists or engineers dealing with semiconductor devices.The aim of this book is to summarize the main experimental results on avalanche and breakdown phenomena in semiconductors and semiconductor devices and to analyze their features from a unified point of view. Attention is focused on the phenomenology of avalanche multiplication and the various kinds of breakdown phenomena and their qualitative analysis.