  In this talk I will summarize some of the work that the CfA group has done to
study the structure of the water masers in the accretion disk of NGC 4258. A
series of 18 epochs of VLBA data taken from 1997.3 to 2000.8 were used for this
study. The vertical distribution of maser features in the systemic group was
found to have a Gaussian distribution, as expected for hydrostatic equilibrium,
with a $\sigma$-width of 5.1 $\mu$as. If the disk is in hydrostatic
equilibrium, its temperature is about 600K. The systemic features exhibit a
small, but persistent, gradient in acceleration versus impact parameter. This
characteristic may indicate the presence of a spiral density wave rotating at
sub-Keplerian speed. A more precise understanding of the dynamical properties
of the disk is expected to lead to a more refined estimate of the distance to
the galaxy.
