  Strongly interacting fermions define the properties of complex matter at all
densities, from atomic nuclei to modern solid state materials and neutron
stars. Ultracold atomic Fermi gases have emerged as a pristine platform for the
study of many-fermion systems. Here we realize a quantum gas microscope for
fermionic $^{40}$K atoms trapped in an optical lattice, which allows one to
probe strongly correlated fermions at the single atom level. We combine 3D
Raman sideband cooling with high-resolution optics to simultaneously cool and
image individual atoms with single lattice site resolution at a detection
fidelity above $95\%$. The imaging process leaves each atom predominantly in
the 3D ground state of its lattice site, inviting the implementation of a
Maxwell's demon to assemble low-entropy many-body states. Single site resolved
imaging of fermions enables the direct observation of magnetic order, time
resolved measurements of the spread of particle correlations, and the detection
of many-fermion entanglement.
