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One of the fundamental questions in modern astronomy concerns the life cycle of molecular material in the universe and addressing how molecular clouds are formed. While this topic is highly complex, absorption spectroscopy of hydrides (molecules or molecular ions with a heavy atom covalently bonded to one or more hydrogen atoms) performed over the past decade have provided a wealth of new information about their use as sensitive tracers of the different phases of the interstellar medium (ISM). Moreover, being light molecules, the fundamental rotational transitions of these hydrides lie at terahertz frequencies, a frequency window for which SOFIA is the only observatory that can provide access that is almost unhindered by atmospheric absorption. In this talk I will introduce HyGAL, a SOFIA Legacy program aimed to address several questions related to the HI-to-H2 phase transition and star-formation in general, using observations of six key hydride molecules- ArH+, OH+, H2O+, SH, OH and CH- and two atomic constituents -C+ and O- within the diffuse ISM.