Abstract: The question of the origin of ultra high energy, > 10^19 eV, cosmic rays (UHECRs) remains unanswered, although experimental searches in the last decade have yielded important results, and insights about the universe at ultra-high energies. I will discuss the interpretation of the most recent measurements of the extensive air-showers produced by UHECRs, and outline current strategies aiming to answer the question of UHECR origin. Emphasis will be given to studies of UHECR arrival directions, searches for single UHECR sources through their secondary gamma-ray and neutrino signatures, and the nascent program, by the Pierre Auger Observatory, of real-time searches for transient UHE emission, as part of multi-messenger monitoring networks.