The biologically important light elements are hydrogen, carbon, nitrogen, oxygen, and sulfur. Each has at least two stable isotopes, and the most abundant isotope in a pair is the lighter: 2H/1H (i.e., D/H), 13C/12C, 15N/14N, 18O/16O, and 34S/32S. Variations in isotope abundances can reveal and quantify processes in which these elements are involved. Such processes include photosynthesis, respiration, evaporation, organic matter turnover, and C, N, and S metabolism. Stable isotopes can also be used in activities as diverse as monitoring pollution events, tracking animals’ food sources, reconstructing past climates, identifying plants’ water sources, and untangling biochemical pathways.