Stellar Disk Truncations: Where do we stand ?

In the light of several recent developments we revisit the phenomenon of galactic stellar disk-truncations. Even 25 years since the first paper on outer breaks in the radial light profiles of spiral galaxies, their origin seems to be still unclear. The two most promising explanations are, that these 'outer-edges' either trace the maximum angular momentum during the galaxy formation epoch, or are associated with global star-formation thresholds. Depending on the true physical nature these outer edges may represent an improved size characteristic (e.g. as compared to D25) and might contain fossil evidence imprinted by the detailed galaxy formation and evolutionary history. We will address several observational aspects: their existence, not only in normal HSB galaxies, but also in LSB and even dwarf galaxies; their detailed shape (the apparent edges are actually not sharp cut-offs as thought before but demarcate a steeper exponential distribution of starlight); their possible association with bars; as well as problems related to the line-of-sight integration for edge-on galaxies (the main targets for truncation searches so far). All together these observations currently prefer the star-formation threshold model, but certainly more work is necessary to implement the truncations as adequate parameters characterising galactic disks