Neutron capture nucleosynthesis during stellar helium burning (s-process) accounts for the production of about half of the elements heavier than iron. Since this process involves mostly stable nuclei along the stability valley, the related nuclear physics data can be determined in laboratory experiments. This allows a quantitative reproduction of the observed abundances. A particularly interesting feature of the s-process is the occurrence of the so-called branchings in the neutron capture chain. Whenever the reaction flow encounters an unstable isotope with a halflife similar to the neutron capture time, competition between (beta) -decay and neutron capture gives rise to such a branching. The resulting abundance pattern can be analyzed in terms of the stellar neutron flux. In some cases the branchings are even sensitive to the temperature and density in the stellar plasma. The current status of this important source of information on the late stages of stellar evolution is reviewed and compared to the yet uncertain stellar models.