This book is a teaching book for advanced undergraduate and/or postgraduate levels. Its primary goal is to link some contemporary applications in chemistry, biology and medicine to known photophysical principles and laser properties. Chapters 1-3 highlight the basic photochemical and photophysical principles associated with electronic states together with a survey of laser systems, timing, and related devices. The following chapters include several techniques and applications in the fields of chemistry, biology and medicine that are based on these basic principles. This approach provides a means of motivating the student to link simple scientific ideas to their potential applications. The discussed techniques were grouped according to the photophysical or laser characteristics that are common among them. For instance, the photophysical properties of internal conversion (ic) and vibrational cascade (vc) constitute the basis of techniques depending on changes in molecular flexibility. We discussed several techniques depending on this phenomenon namely the detection of salmonella by the commercially available 4-methyl umbelliferyl caprilate (MUCAP) reagent, the modification of fingerprints upon metal ion complexation, DNA quantification by flexible fluorescent stains. The heat evolution associated with vc process is the basis of thermal lensing techniques as well as the laser welding of some biological tissues. Molecular fluorescence, another photophysical deactivation pathway, is the workhorse technique in medical diagnosis and analytical applications. The capabilities of fluorescence techniques were largely modified by applying time-resolved phenomena as well as polarized fluorescence. Applications