Cosmological hydrodynamics simulations are increasingly able to reproduce galaxies like those we observe in nature. However, the detailed structure of the circumgalactic medium (CGM), the tenuous gas distributed throughout the galactic halo, remains an unresolved challenge. As the interface between star formation, feedback, galactic gas accretion, and acting as a reservoir hosting half the baryons in a galaxy, the CGM is the key to understanding what drives galactic evolution. Due to its low density, observations of the CGM are difficult, so developments in the field rely heavily on theoretical modeling. I will discuss why the CGM is historically so difficult to model, how new generations of simulations (e.g., FIRE, Tempest) are now beginning to shed light on its behavior, and what these models tell us about galactic feedback and accretion, thermal instability, cosmic ray diffusion, and fast radio bursts.
