Molecular Differentiation of Starless Cores by Dr. Mario Tafalla (OAN, Spain)   Dense cores, the simplest units of star formation, have been recently found to be strongly differentiated in their chemical composition. Molecules like CO and CS dissapear toward the core centers, most likely due to depletion, while other species, like N2H+ and ammonia remain in the gas phase. As molecules are used to determine the core physical properties and to search for evidence of infall, characterizing the pattern of chemical differentiation in dense cores has become an urgent need. I will review recent observations and modeling of the chemical composition of starless cores, and discuss how understanding their chemical structure can help us understand the process of core formation.