The galactic distributions of the stable isotopic species of hydrogen, carbon, nitrogen, oxygen, sulfur, and silicon have been derived from the study of interstellar molecules which contain them. The resulting observational framework appears consistent with our general understanding of stellar evolution and nuclear processing in the galaxy. The greater amount of star formation that has taken place near the center of our galaxy is reflected in an enhanced abundance of processed material, notably carbon-13, in this region. Significant differences between abundances of isotopes in the solar system and corresponding values in interstellar space are also evident. Although some of these differences merely reflect the results of additional stellar processing since the condensation of the presolar nebula some 5 billion years ago, others suggest that a significant amount of atypical nuclear material is associated with the solar system.