Maser Astrometry: from Galactic Structure to Local Group Cosmology

This review summarizes current advances in astrometry of masers as they pertain to large-scale Galactic structure and dynamics and Local Group cosmology. Parallaxes and proper motions have now been measured for more than 60 massive star forming regions using the Japanese VERA array, the EVN and the VLBA. These results provide ``gold standard'' distances and 3-dimensional velocities for sources across the Milky Way, revealing its spiral structure. Modeling these data tightly constrains the fundamental parameters of the Milky Way: R₀ and Θ₀. Proper motions of Local Group galaxies have been measured, improving our understanding of the history and fate of the Group.