Ultra-high-energy cosmic rays (UHECR) are the most energetic known particles of the Universe, being accelerated to energies up to 7 orders of magnitude higher than those achieved in human-made accelerators. Their origins, however, remain an intriguing puzzle even decades after their discovery. In this seminar, the current status of this puzzle will be discussed. The state-of-the-art experiments will be presented as well as a compilation of relevant data collected so far. The power and limitations of phenomenological studies to decipher this question will be discussed, focusing on models to estimate the role of local UHECR sources.