Evolved stars in the Asymptotic Giant Branch (AGB, a crucial phase in the late evolution of most stars, including solar-type stars) present copious mass-loss processes, which finally lead to the ejection of most of the stellar mass and drive the end of that phase and the formation of planetary nebulae around white dwarf stars. During the AGB phase, the ejection is significantly isotropic, leading to wide spherical shells that are radially expanding with moderate velocities. The crucial regions to study these phenomena are the very inner circumstellar layers, where the mass loss process is actually taking place. These regions can be observed with very high resolution, better than one milliarcsecond, thanks to VLBI observations of maser lines. I will review recent studies in this field of our group, stressing the interest of simultaneous observations of different lines and of high accuracy astrometry. We have recently found a surprising very fast variability in some objects, future VLBI studies of this phenomenon will be discussed.