  Imaging the bright maser emission produced by several molecular species at
centimeter wavelengths is an essential tool for understanding the process of
massive star formation because it provides a way to probe the kinematics of
dense molecular gas at high angular resolution. Unimpeded by the high dust
optical depths that affect shorter wavelength observations, the high brightness
temperature of these emission lines offers a way to resolve accretion and
outflow motions down to scales as fine as $\sim$1-10 au in deeply embedded
Galactic star-forming regions, and at sub-pc scales in nearby galaxies. The
Next Generation Very Large Array will provide the capabilities needed to fully
exploit these powerful tracers.
