  The status of solutions to the ultra-high energy cosmic ray puzzle that
involve particle physics beyond the standard model is reviewed. Signatures and
experimental constraints are discussed for most proposals like the Z burst
model and topological defects (both allowed only as subdominant contribution),
supermassive dark matter (no positive evidence from its key signatures galactic
anisotropy and photon dominance), strongly interacting neutrinos or new
primaries (no viable models known), and violation of Lorentz invariance
(possible).
