Neutral hydrogen 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. Recent high-resolution HI data for external galaxies are revealing the features and topology of the entire neutral ISM, which here are considered on scales of 10 - 1000 pc. I focus on two principal candidates for HI structuring: mechanical feedback from massive stars, and turbulence. While confirmation for both these processes exists, it remains unclear how these mechanisms yield the global, steady-state, scale-free HI properties that are observed. Understanding the formation of filamentary structure may be key in resolving these puzzles.