Introduction
============

The sample of identified field stars with \[Fe/H\][^1]$<-2.4$ has increased by more than an order of magnitude in the last decade. Because the elements in the atmospheres of these stars have been produced in a small number of nucleosynthetic events, abundance determinations can provide direct tests of model yields from different nuclear processes. Even in the early work on very metal-poor stars it quickly became clear that there were subclasses of objects with very high \[heavy-element/Fe\] ratios that could be traced to specific nucleosynthetic origins (e.g., McWilliam [et al.]{} 1995).

The elements heavier than the iron peak are made through neutron capture via two principal processes: the r-process and the s-process (Burbidge [et al.]{} 1957). The r-process (for rapid process) occurs when neutrons are added much more rapidly than the $\beta$ decay times of the relevant nuclei. The site or sites of the r-process are not known, although suggestions include the $\nu$-driven wind of Type II SNe (e.g., Woosley & Hoffman 1992; Woosley [et al.]{} 1994) and the mergers of neutron stars (e.g., Lattimer & Schramm 1974; Rosswog [et al.]{} 2000). The s-process occurs when neutrons are added more slowly and $\beta$ decays, changing neutrons to protons, keep 