Abstract Oxygen, carbon, and hydrogen are the most abundant elements in mineralizing fluids, and their isotope ratios provide a powerful tool for deciphering the complex histories of mineralizing systems (see Ohmoto, 1986; Kerrich, 1987; Taylor, 1987; Hebert and Ho, 1990). In particular, stable isotope ratios can be used to constrain theories of fluid sources, pathways, and fluxes, mechanisms of mineral reaction and exchange, and thermal evolution (see Valley, 1986; Bickle and McKenzie, 1987; Baumgartner and Rumble, 1988; Nabelek, 1991; Eiler et al., 1993; Skelton et al., 1995). Sulfur isotopes have special importance for the genesis of sulfide ores and are discussed in separate chapters of this volume (McKibben and Riciputi, 1998; Shanks et al., 1998). Microanalysis of boron isotope ratios was recently reviewed by Hervig (1996). This chapter will review studies of the stable isotopes of oxygen, carbon, and hydrogen ( 18 O, 17 O, 16O , 13 C, 12 C, 2 H (D), and 1 H (H), referred to collectively as stable isotopes) with emphasis on recent work that has attained accuracy in the 1 per mil range for δ 18 O or δ 13 C (10‰ for δD) that is necessary for research on terrestrial samples. The vast majority of all stable isotope ratio analyses have been made of 1 to 100 mg samples of whole-rock powder or mineral separate. Commonly, analyses have been made of splits, prepared from much larger samples with the implicit assumption that measured compositions are representative