This book deals with lock-in thermography (LIT) as a special active dynamic variant of the well-known IR thermography. It enables a much improved signal-to-noise ratio (up to 1000x) and a far better lateral resolution compared to steady-state thermography. The book concentrates on applications to electronic devices and materials, but the basic chapters are useful as well for non-destructive evaluation. Various experimental approaches to LIT are reviewed with special emphasis to different available commercial LIT systems. New LIT applications are reviewed, like Illuminated LIT applied to solar cells , and non-thermal LIT lifetime mapping. Typical LIT investigation case studies are introduced.