The origin of the heavy elements: The s process

The heavy elements with 2 greater than or equal to 30 are made in about equ al quantities by neutron capture reactions during stellar He burning and pr esumably in supernovae. This contribution deals mainly with the slow neutro n capture (s) process which is responsible for about one half of the abunda nces in the mass region between Fe and Bi. The slow time scale implies that the reaction path of this process involves mostly stable isotopes which ca n be studied in detail in laboratory experiments. Based on these data, the quantitative interpretation of the natural abundances provides an exciting possibility for exploring a variety of problems related to stellar and Gala ctic evolution. The p process, which provides a wry small but significant a dmixture to many of the s abundances, has recently attracted increasing int erest as a possibility for supernova studies.