energy CR propagation is diffusive in the turbulent galactic magnetic field. Sources of these CRs are obscured because of that. On the other hand, since the gyroradius of a particle with charge $Z$ and energy $E_{EeV} = E/$EeV is $r_g \approx (E_{EeV}/ZB_{\mu G}~$kpc), and the characteristic galactic magnetic field is a few $\mu$G, confinement of CRs within the galaxy is limited to energies below $\sim 1 E_{EeV}/Z$. Diffusive propagation within the galaxy should break down well below that energy. Indeed, around the ankle some enhancement in the CR flux is reported towards the galactic center (Hayashida, 1999), so we may have seen direct indications of their galactic origins. At higher energies, however, the data are consistent with isotropic detections (Takeda, 1999; Uchihori, 2000). Unless the UHECRs are highly charged ($Z>>1$) this almost assures their extragalactic origins.

Some Propagation Issues
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Before discussing the physics of various acceleration scenarios it is helpful to establish a clear picture of what general limits can be placed on the locations of possible extragalactic sources of the UHECRs. The most frequently discussed constraint on the origins of the observed particles comes from the large energy losses by hadrons through inelastic collisions with photons. This is 