Hi as a Probe of Structure in the Interstellar Medium of External

This review presents a perspective on recent advances in understanding neutral ISM structure in external galaxies. Hi is a fundamental probe of galactic baryonic material, and its structure and distribution offer vital signatures of dynamical and evolutionary processes that drive star formation and galaxy evolution. New, high-resolution Hi data cubes for external galaxies now reveal the features and topology of the entire neutral ISM, which here are considered on scales of 10 – 1000 pc. I focus on the two principal candidates for Hi structuring, mechanical feedback from massive stars and turbulence; other mechanisms are also considered , especially with respect to supergiant shells. While confirmation for both mechanical feedback and turbulent processes exists, it remains unclear how these mechanisms yield the global, steady-state, scale-free Hi properties that are observed. Understanding the formation of filamen-tary structure may be key in resolving these puzzles. New Hi surveys of nearby galaxies, combined with further theoretical studies, promise continuing important advances.