How do stars manage to form within low-density, HI-dominated gas? Such
environments provide a laboratory for studying star formation with physical
conditions distinct from starbursts and the metal-rich disks of spiral galaxies
where most effort has been invested. Here we outline fundamental open questions
about the nature of star formation at low-density. We describe the wide-field,
high-resolution UV-optical-IR-radio observations of stars, star clusters and
gas clouds in nearby galaxies needed in the 2020's to provide definitive
answers, essential for development of a complete theory of star formation.