Water is the most important element for a life. Scientists have been doing thorough investigations on the properties of water while engineers have - ploited water in various industrial processes that aim at better optimization of the quality of industrial products. The knowledge of the physico-chemical properties of various liquids is extremely important in the ?eld of life s- ences. Recently, the study of liquids containing nanoparticles has been gr- ing,sincee. g. themoderndevelopmentofnewdrugsbybioa?nityassays,and drug delivery is based more and more on the exploitation of nanoparticles in liquid phase. Optical metrology in context of drugs development is becoming an important tool in high-throuhgput screening of potential molecules. - fortunately, process liquids in life sciences and in industrial environments are usually ”ill-behaving” and it is usually di?cult to monitor their condition. Fortunately,opticalsensingbasedonre?ectometrycanbeexploitedininsp- tion of the quality of transparent and turbid liquids. This book is exceptional in the sense that it introduces the theory and practical re?ectometry between same covers. We recommend this book for beginners and professionals who are doing research or working with a variety of liquids either in academic or engineering societies. The book is helpful for physicists, chemists and en- neers since it covers topics related to basic physics and optics, optical spectra analysis, optical metrology and practical building of re?ectometers. The authors wish to express their cordial thanks to Professor Rauno Aulaskari for helping in the formulation of Appendix B and to Dr. Chun Ye for his critical comments. We will also remember with warmth Mrs.