Observational Constraints on the Acceleration Discrepancy Problem

I review the data relating to the appearance of the missing mass problem at a particular acceleration scale. Rotation curves are examined in detail, with emphasis on the empirical connection between baryonic and total mass distributions. Other lines of evidence are also considered, including disk stability, dwarf spheroidals, giant ellipticals, clusters of galaxies, the microwave background, BBN, and the growth of large scale structure.