  We report heavy electron behavior in unconventional superconductor
YFe$_2$Ge$_2$ ($T_C \,{=}\, 1.2$ K). We directly observe very heavy bands
($m_\mathrm{eff}\sim 25 m_e$) within $\sim$10 meV of the Fermi level $E_{F}$
using angle-resolved photoelectron spectroscopy (ARPES). The flat bands reside
at the X points of the Brillouin zone and are composed principally of $d_{xz}$
and $d_{yz}$ orbitals. We utilize many-body perturbative theory, GW, to
calculate the electronic structure of this material, obtaining excellent
agreement with the ARPES data with relatively minor band renormalizations and
band shifting required. We obtain further agreement at the Dynamical Mean Field
Theory (DMFT) level, highlighting the emergence of the many-body physics at low
energies (near $E_F$) and temperatures.
