Unravelling the Chemical Structure of Young Stellar Objects with ALMA

The importance of ALMA to quantitatively trace the evolution of gas and dust from interstellar clouds to planetary systems is discussed and illustrated with recent observational studies of molecules in lowand high-mass young stellar objects. Line surveys using single-dish submillimeter telescopes reveal large chemical variations between different sources, which are likely related to their evolutionary state. A chemical scenario is presented in which molecules produced by grain-surface chemistry during the cold collapse phase are returned to the gas once the young star has formed, leading to an active high-temperature gas-phase chemistry. Both thermal evaporation and release of icy mantles in shocks plays a role. Current observational facilities lack the spatial resolution and image quality to distinguish these different physical and chemical regimes. Exploratory observations of the excitation and distribution of molecules in circumstellar disks are presented, but only ALMA can study the chemistry in the planet-forming zones of circumstellar disks.