Nucleosynthesis of elements heavier than the iron group by neutron capture on both slow and fast time scales is evaluated. The s-process calculations of Clayton, Fowler, Hull, and Zimmerman (1961) have been revised to include more recent experimental results on abundances and neutron capture cross- sections. The solar-system s-process abundances indicate a history of neutron exposure distributions characterized by decreasing probability of high integrated flux; an exponential exposure distribution is extracted. Estimates are made of the s-process contribution to each isotopic abundance; a table gives the amounts of elements produced by each process in the solar-system material. The r-process calculations are carried out using a semi-empirical atomic-mass law to determine neutron-binding energies and betadecay probabilities. The solar-system r-process material has probably been synthesized in two distinct types of environments, e.g., one of about 4 sec duration with temperature 2.4 X 10 K and neutron density 5 X tO , and the other of the same or longer duration with temperature 1.0 X 10 K and neutron density 3 X 1O both of these environments could be found in an object with mass 10 Mo.