  I briefly review the growing evidence that thick stellar disks surround most
edge-on disk galaxies. Recent studies show that these extragalactic thick disks
have old ages, low metallicities, long scale lengths, and moderately flattened
axial ratios, much like the thick disk of the Milky Way. However, the
properties of thick disks change systematically with the mass of the galaxy.
The thick disks of low mass galaxies are more prominent and somewhat more
metal-poor than those surrounding massive disk galaxies. Given the strong
evidence that thick disks are fossils from an early epoch of merging, these
trends place tight constraints on the early assembly of disk galaxies. The data
suggest that the vast majority of the accretion onto the early disk was
gaseous, but that the majority of thick disk stars were directly accreted.
