Red supergiants (RSGs) are a short-lived but important stage in the lives of massive stars. Such stars are among the most optically luminous, and they have the largest diameters of any stars. For the past few years we have been measuring the physical properties of RSGs in the galaxies of the Local Group using both optical spectroscopy and K-band photometry, aided by the new generation of "MARCS" stellar atmospheres. Our work has succeeded in removing the previous discrepancy with stellar evolutionary theory, while uncovering a number of surprises, including the fact that these stars have significant extra extinction due to circumstellar dust. We have also found that RSGs may be the primary source of dust in starburst galaxies, and in galaxies at large look-back times. In addition, we have identified a new class of spectral/photometric variable RSGs at low metallicities, stars which appear to be in a highly unstable evolutionary phase, possibly nearing the end of their lives.