Abstract: The origin of ultra high energy cosmic rays (UHECRs) remains unknown despite decades of experimental and theoretical research; their discovery will bring to light the nature of the Universe's most violent accelerators. In this talk I will discuss the observable signatures of UHECRs that can be used to constrain the nature and distribution of their yet unknown sources, focusing on the constraints imposed by the arrival direction distribution of the highest energy cosmic rays detected at the Pierre Auger observatory in Argentina. Constraints on the sources of UHECRs may also come from the secondary particles (gamma-rays and neutrinos) that UHECRs produce during their propagation. I will present the results of a search for the signatures of UHECR acceleration in the gamma-ray spectra of blazars and discuss the implications for the detectability of such signatures with current and upcoming gamma-ray instruments.