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Mega-masers are aptly named as their inferred luminosities exceed those of galactic source by a factor $\gt 10^6$. Their high surface brightnesses and very compact nature make them ideal to study with Very Long Baseline interferometry, providing geometric distances of galaxies past 100 Mpc, well into the Hubble flow.  In \S\ref{sec:OH}, I presented the current state of research on OH mega-masers. Survey results show that the process is linked to a stage of major galaxy evolution, where extreme starburst activity is occuring near one of the merging nuclei. OH mega-masers have been used to directly measure the magnetic fields in these extreme starburst regions. With upcoming improvements, it is predicted that OH mega-masers can be used to probe the merger rates of galaxies up to $z\sim4$. This is shown in Figure XXX, \ref{fig:oh_limits},  where \citet{darling2002_lumfunc} have predicted the sensitivity limits of several new or future instruments. I summarized the current efforts for using H$_2$O mega-masers to constrain fundamental physical properties in \S\ref{sec:h2o_mm}. Detections of H$_2$O mega-masers show that they arise from parsec-scales from AGN, and are only detected for certain classes of AGN, namely LINERs and Sy2 galaxies. H$_2$O mega-masers are being used to measure SMBH masses to high precision, strengthening the evidence that SMBHs exist. The direct detection of Keplerian circumnuclear disks about AGN allow for distances of galaxies past 100 Mpc to be measured. This provides an independent measure of the Hubble Constant, and with future work, is expected to reach similar uncertainty levels of other meausurements.