Gas within and around galaxies is commonly multiphase, that is, vastly different temperates exists cospatially. This is for instance the case in the interstellar, circumgalactic or intracluster medium but also in galactic winds or in the solar corona. Due to the temperate and density contrasts as well as the different physical mechanisms at play the dynamics of such systems is difficult to model and understand (but nevertheless crucial for, e.g., the galactic ecosystem). In this talk, I will present some theoretical and numerical results which highlight under which conditions the phases can co-exist, what sets the mass transfer rate between them, and what are typical morphologies. In particular, I will show (and try to reason why) that cold clumps follow a Zipf's dN/dm ? m^-2 law which is also common in other astrophysical (IMF, dust mass, ...) and non-astrophysical contexts. Time provided, I want to switch gears and show how we can use the Lyman-alpha line of neutral hydrogen to constrain such systems and in particular what the emergent spectra can tell us about anisotropic gas distributions which is, e.g., relevant for the escape of ionizing photons.