Book excerpt: This handbook looks into the latest scientific achievements in the observation, technical development, theory, and application of isotopologues. Isotopes have been used to trace material cycles in the solar system, planets, atmosphere, oceans, terrestrial environment, biosphere, and also the interior of living organisms. Isotopic research has developed from the determination of the bulk content of a single isotope ratio to multi-isotope, mass independent fractionation (MIF) of oxygen and sulfur isotopes, doubly isotope-substituted species, and site-specific isotopic signatures at natural isotopic abundance levels using several novel analytical techniques. The study of isotopologues has an increasing number of applications in many scientific fields ranging from earth and planetary sciences, environmental studies including climate change to life sciences relevant to many areas of interest for human activities such as agriculture, food authentication, pharmacology, and health care diagnosis. This handbook is structured so that is appropriate for both graduate students and professionals in diverse scientific and engineering communities devoted to related fields.