This book provides a systematic introduction to the physics behind measurements on plasmas. It develops from first principles the concepts needed to plan, execute, and interpret plasma diagnostics. The book is therefore accessible to graduate students and professionals with little specific plasma physics background, but is also a valuable reference for seasoned plasma physicists. Most of the examples are taken from laboratory plasma research, but the focus on principles makes the treatment useful to all experimental and theoretical plasma physicists, including those interested in space and astrophysical applications. This second edition is thoroughly revised and updated, with new sections and chapters covering recent developments in the field. Specific areas of added coverage include neutral-beam-based diagnostics, flow measurement with mach probes, equilibrium of strongly shaped plasmas and fusion product diagnostics.