I will discuss the cosmological origin of stellar and gaseous disks in the Milky Way (MW) and similar-mass galaxies, using the FIRE (Feedback in Realistic Environments) cosmological zoom-in simulations. First, I will discuss the formation history of the MW and how it depends on the Local Group environment, and I will show evidence that the MW is unusually thin and dynamically cold relative to nearby disk galaxies. Then, I will discuss how stellar and gaseous disks dynamically settle over cosmic time, and how stars are dynamically heated within the disk after they form. I will compare the relative importance of disk setting and post-formation dynamical heating in determining the observable relation between stellar dynamics and stellar age today. Finally, I will discuss the physical drivers that regulate the onset of disk formation in the early Universe.
