Abstract: Particles near $10^{20}$ eV are the most energetic particles known to us in the universe, also called ultra high energy cosmic rays. Their observations have led us to build the largest detector systems in the world, in the South the Auger air-shower array, and in the North the Telescope Array, and perhaps soon Auger-North. With these and earlier arrays events have been detected of an energy up to $3 , 10^{20}$ eV, which is a macroscopic energy. There have been two predictions: one that due to interaction with the microwave background the spectrum should show a turnoff near $5 , 10^{19}$ eV; a turn-off has been confirmed by two experiments, HiRes and Auger. Second, that active galactic nuclei, possibly radio galaxies, should be the accelerators, based on the non-thermal optical spectra of knots and hot spots in radio galaxies; this is now tentatively confirmed by Auger, but contradicted by HiRes. I will go through some fundamental problems with the predictions, which teach us about active galactic nuclei and starburst galaxies. Apart from differentiating various remaining options, such as gamma ray bursts, how to generate these particles, and their source population, there is one major difficulty: the lack of understanding of the cosmological web of magnetic fields, which may influence the propagation of high energy particles; here it is especially important to understand the role of our local cosmic neighborhood and a possible galactic magnetic wind. It appears from MHD simulations that magnetic scattering leads to a steep distribution function of scattering angles of the deviation from a straight line path for the arriving particles, and also to a substantial delay time distribution. I will list and debate the merits of the closest candidate sources, Cen A, Vir A and For A. I will discuss the observational and theoretical limits for an exemplary set of models, the predictions like chemical abundances, that result from these models, and how present and future observations will test our conclusions, especially with the the Auger Array, the Telescope Array (TA), the neutrino observatory IceCube, the TeV Cherenkov $gamma$-ray telescopes, and the future space observatory EUSO. We face a number of exciting challenges for plasma physics, particle physics, cosmology, astronomy, and may attain better tools for our deep understanding of matter.