Circumstellar disks around young protostars are the birthplaces of
planets, and understanding their formation is key to unraveling the
early stages of planet formation. Recent theoretical work has
highlighted the critical role of non-ideal magnetohydrodynamic (MHD)
effects, particularly ambipolar diffusion, in enabling the formation
and growth of sizable disks in magnetized dense cores. In this talk, I
will present observational evidence supporting these theories, showing
that models incorporating efficient ambipolar diffusion can
successfully reproduce the observed disk size distribution, whereas
other mechanisms such as turbulence, magnetic field strength, and
magnetic field-rotational axis misalignment alone cannot explain disk
formation. I will also introduce key observational signatures of
ambipolar diffusion and share our recent results hinting at efficient
ambipolar diffusion in protostellar sources.