Abstract
Most elements heavier than iron in the Universe are produced through neutron captures. The solar system abundance distribution indicates that they were created primarily in two nucleosynthetic processes, the slow and the rapid processes. Here, we cover the fundamental aspects of the s-process. The ever-growing amount of information gathered from astronomical observations and analyses of stardust grains recovered from meteorites provides tight constraints on the mechanisms of the s-process and where it occurs. Comparison of chemical abundance patterns with stellar model predictions clearly identifies evolved low-mass and massive stars as the s-process astrophysical sites.