Stellar spectra can provide useful constrain on stellar evolution models but also on Galactic population. After presenting the abundance analysis technique, I will present results concerning two important populations of the Galaxy. The first one concerns the Galactic disks from which we are able to infer their formation history by looking at the carbon and nitrogen abundances in the giant stars. The second population concerns the halo of the Galaxy and in particular the so-called carbon enhanced metal-poor stars. The measurement of carbon, nitrogen and other neutron-capture elements in those peculiar stars allows to study in details the evolution of the primordial stages of the Galaxy. However, I will also show our current limitations in terms of evolution and nucleosynthesis modelling of red giant branch and asymptotic giant branch stars.