The present book focuses on the capabilities of laser photoacoustic spectroscopy method in high impact and new frontier of modern applications: medical diagnosis based on exhaled breath analysis with special importance on the theory, application of the simultaneous measurements of ethylene and ammonia absorptions and the study of exhaled respiration of subjects with renal failure together with breath test analysis of one prevention domain: smoking. This book consists of 4 chapters plus appendices, the first two chapters are focussed on general introduction to human breath biomarkers and the instrumental aspects of the techniques which makes it useful to readers whose main interest is photoacoustic spectroscopy and particularly breath air analysis. The technique developed in this book ensuring the advantages of health state assessment by monitoring the evolution of gaseous biomarkers in human body, impossible to achieve with current techniques. The book also, represents a potentially interesting to researchers and specialists with the application of biomedical optics domain and particularly life sciences area.