Molecules on a Space Odyssey

New observational facilities at infrared and submillimeter wavelengths have led to enormous advances in our understanding of the formation of stars and planets deep inside molecular clouds. The chemistry responds to the changes in physical conditions by creating different species in the gas and solid state. Molecules are therefore excellent probes of the different phases of star formation. This talk will follow the molecules on their journey from the tenuous clouds to planet-forming disks around young stars and eventually comets in our own solar system. Special emphasis will be placed on results obtained with the Infrared Space Observatory (ISO), which provided the first opportunity for spectroscopic observations over the complete 2.4--200 microns wavelength range above the atmosphere. These include direct observations of the rotational transitions of the dominant molecule H2, the main ingredient for giant planet formation. Other topics concern the formation and evaporation of interstellar ices, unexpected new features due to crystalline silicates and the wealth of data on gas-phase H2O.