  Ultra-High Energy (UHE, $>$0.1\,PeV) $\gamma$-ray Astronomy is rapidly
evolving into an expanding branch of the $\gamma$-ray astronomy with the
surprising discovery of 12 PeVatrons and the detection of a handful of photons
above 1 PeV. Nearly all known celestial object types that have emissions in the
TeV band are found also emitting UHE photons. UHE $\gamma$-rays have a
well-defined horizon inside our galaxy due to the absorption of infrared and
cosmic microwave backgrounds in the universe. With the last 30 years,
traditional cosmic ray (CR) detection techniques allow the detection of UHE
$\gamma$-rays, and opened up the last observation window. For leptonic sources,
UHE radiation is in the deep Klein-Nishina regime which is largely suppressed.
Therefore UHE $\gamma$-ray detection will help to locate and identify hadronic
radiation sources, tracing the historic pursuit for the origin of CRs around
the knee of the spectrum. The Crab Nebula is again the focus of attention with
measured photon emissions above 1\,PeV. In the absence of hadronic processes,
this may indicate the existence of an extreme accelerator of e$^+$/e$^-$.
Utilization of the CR extensive air shower detection techniques broadens the
field of view of the source observations, enabling the measurement of UHE
radiation surrounding the sources. These observations can probe the particle
propagation inside and outside the accelerators and the subsequent
injection/escape into the interstellar medium.
