  Elemental abundance ratios as tracers of stellar populations are discussed
with emphasis on F, G, and K stars providing a `fossil' record of the chemical
evolution of the Galaxy. Most abundance studies have been based on homogeneous
1D model atmospheres and the assumption of local thermodynamic equilibrium
(LTE), but recent works have shown that 3D non-LTE corrections can change the
derived trends of abundance ratios as a function of stellar metallicity very
significantly. However, when comparing stars having similar effective
temperatures, surface gravities, and metallicities, 3D non-LTE corrections tend
to cancel out. When applying such a differential approach to stars in the
Galactic disk, bulge, and halo, abundance ratios like C/O, Na/Fe, alpha/Fe,
Cu/Fe, Ba/Y, and Eu/Ba point to the existence of multiple discrete populations
in each of these Galactic components.
