The origins of the most energetic particles in the Universe has been a long-standing puzzle. In the quest to identify their sources, it is essential to understand how these particles are accelerated, how they can escape the sites wherein they are produced, and which roads they can take on their journey to Earth. The multimessenger framework is a powerful tool for unveiling the Universe at the highest energies.
Ultra-high-energy cosmic rays play a central role in this context because their interactions with matter and radiation fields are responsible for the production of copious amounts of other messengers, including high-energy photons and neutrinos. In this talk I will focus on the triad of (ultra-)high-energy messengers: cosmic rays, neutrinos, and gamma rays. I will first briefly discuss the state of affairs in this field, reviewing some key theoretical developments and experimental results. I will then present the ingredients required for building theoretical models to interpret the measurements. Emphasis will be given in modelling the propagation of these messengers, including their interactions with matter and radiation fields, and with the poorly-understood cosmic magnetic fields. Finally, I will discuss the prospects for building a unified self-consistent model of the Universe at ultra-high energies and the implications such a model would have for astrophysics, cosmology, and fundamental physics.