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Ultra-high energy cosmic rays (UHECRs) are the most energetic particles ever detected, reaching energies up to more than ten million times the beam energy of the Large Hadron Collider (~10^8 TeV). As such, UHECRs offer an unparalleled opportunity to probe fundamental physics at energy scales that are far out of reach for terrestrial accelerators. However, even after more than a century of study, the nature and origin(s) of UHECRs remain elusive. The past twenty years of UHECR experiments have brought about significant advances in our understanding of UHECRs and in using them for particle physics measurements, but many challenges and open questions remain. In this talk, I will provide an overview of UHECR physics, including highlights of particle physics measurements and searches for beyond-Standard-Model physics that are being performed by UHECR experiments. I will also set the stage for the next-generation UHECR facilities that will step up to the challenges of unraveling the mysteries of UHECRs.