The aim of this paper is to give a review of our knowledge of the chemical composition of young groups of stars in the Galaxy, i.e. open clusters and associations with ages less than about 109 years. In particular I shall discuss if abundance differences between these groups of stars do occur, and how large these possible differences can be. Such information is important in many astronomical fields. Thus, abundance differences between star clusters complicate the determination of their distances by ZAMS-fitting, and make the distance scale in the Universe more uncertain. Comparisons between theoretical isochrones and cluster sequences in color-magnitude diagrams with the purpose of testing the theory of stellar structure and evolution are also much more difficult if differences in abundances have to be taken into account. On the other hand the determination of abundances of clusters and associations as a function of their ages and places of formation give important information on the chemical evolution of our galaxy, in particular because abundances, ages and space velocities for clusters and associations can be determined much more accurate than for single stars.