This book is based on lectures given at the Ruhr-University of Bochum for graduate students and postgraduates starting in plasma physics, but it is as well directed at established researchers being newcomers to spectroscopy and needing quick access to the diagnostics of plasmas ranging from low- to high-density technical systems at low temperatures, as well as from low- to high-density hot plasmas. Basic ideas and fundamental concepts are introduced as well as typical instrumentation from the X-ray to the infrared spectral regions. Examples, techniques and methods illustrate the...