a1 ICE(CSIC)-UB/IEEC, Barcelona (Spain)
a2 IAA(CSIC), Granada (Spain)
a3 Harvard-Smithsonian, CfA, Cambridge (USA)
a4 IAUNAM, México D.F. (México)
a5 UB/IEEC, Barcelona (Spain)
VLBI multi-epoch water maser observations are a powerful tool to study the gas very close to the central engine responsible for the phenomena associated with the early evolution of massive protostars. In this paper we present a summary of the main observational results obtained toward the massive star-forming regions of Cepheus A and W75N. These observations revealed unexpected phenomena in the earliest stages of evolution of massive objects (e.g., non-collimated “short-lived” pulsed ejections in different massive protostars), and provided new insights in the study of the dynamic scenario of the formation of high-mass stars (e.g., simultaneous presence of a jet and wide-angle outflow in the massive object Cep A HW2, similar to what is observed in low-mass protostars). In addition, with these observations it has been possible to identify new, previously unseen centers of high-mass star formation through outflow activity.