  The abundance patterns of metal-poor stars provide us a wealth of chemical
information about various stages of the chemical evolution of the Galaxy. In
particular, these stars allow us to study the formation and evolution of the
elements and the involved nucleosynthesis processes. This knowledge is
invaluable for our understanding of the cosmic chemical evolution and the onset
of star- and galaxy formation. Metal-poor stars are the local equivalent of the
high-redshift Universe, and offer crucial observational constraints on the
nature of the first stars. This review presents the history of the first
discoveries of metal-poor stars that laid the foundation to this field.
Observed abundance trends at the lowest metallicities are described, as well as
particular classes of metal-poor stars such as r-process and C-rich stars.
Scenarios on the origins of the abundances of metal-poor stars and the
application of large samples of metal-poor stars to cosmological questions are
discussed.
