This book describes the techniques and instrumentation of thermal lens spectrometry. Also, includes parameters affecting thermal lens phenomenon, noise and source of errors, advantages and disadvantages. By combining TLS with a microscope, thermal lens microscope is developed for sensitive detection in microspace. The detection limit is about single molecules in a very small volume. One of the most important applications is the various analytical fields, such as immunoassay, environmental analysis, and single-cell analysis on microchips.