The first edition of this book was published in 1973, the second, totally rewritten, followed 7 years later in 1980. Because the field of stable isotopes is still grow ing and exerting an increasing influence on geosciences in general, it seems to be necessary, after a further 7 years, to revise the edition again accordingly. Not only has the previous edition been updated, but two completely new chapters on the isotopic composi tion of mantle-derived material and on the isotopic composition of the ocean during the geologie past, have been added. The references concentrate on re cent literature. In some cases, older references have been omitted to save space. I do not intend to underrate the value of older publications, but only to keep the reference list - al ready very voluminous in relation to the totallengt- from becoming even larger. An early draft has been reviewed by Russell Harmon and Alan Matthews. John Valley has se nt me a preprint of an artic1e on metamorphie rocks. To all three of them I owe my deepest thanks.