Water maser kinematics in massive star-forming regions: Cepheus A and
  W75N
  VLBI multi-epoch water maser observations are a powerful tool to study the
dense, warm shocked gas very close to massive protostars. The very high-angular
resolution of these observations allow us to measure the proper motions of the
masers in a few weeks, and together with the radial velocity, to determine
their full kinematics. In this paper we present a summary of the main
observational results obtained toward the massive star-forming regions of
Cepheus A and W75N, among them: (i) the identification of different centers of
high-mass star formation activity at scales of 100 AU; (ii) the discovery of
new phenomena associated with the early stages of high-mass protostellar
evolution (e.g., isotropic gas ejections); and (iii) the identification of the
simultaneous presence of a wide-angle outflow and a highly collimated jet in
the massive object Cep A HW2, similar to what is observed in some low-mass
protostars. Some of the implications of these results in the study of high-mass
star formation are discussed.
