  The presence of radial truncations in stellar disks is reviewed. There is
ample evidence that many disk galaxies have relatively shaprt truncations in
their disks. These often are symmetric and independent of the wavelength band
of the observations. The ratio of the truncation radius R_{max} to the disk
scalelength h appears often less then 4.5, as expected on a simple model for
the disk collapse. Current samples of galaxies observed may however not be
representative and heavily biased towards disks with the largest scalelengths.
Many spiral galaxies also have HI warps and these generally start at the
truncation radius of the stellar disk. The HI surface density suddenly becomes
much flatter with radius. In some galaxies the start of the warp and the
position of the disk truncation radius is accompanied by a drop in the rotation
velocity. In the regiosn beyond the dis truncation in the HI layer some star
formation does occur, but the heavy element abundance and the dust content are
very low. All evidence is consistent with the notion that the outer gas parts
of the disks constitute recently accreted material, at least accreted after
formation of what is now the stellar thin disk. Although various models exist
for the origin of the truncations in the stellar disks, this at present remains
unclear.
