  Planetary Nebulae (PNe) evolve from Asymptotic Giant Branch (AGB) stars after
a brief post-AGB phase. Water maser emission is characteristic of oxygen-rich
AGB stars, is observed in post-AGB stars and, unexpectedly, has been detected
in three PNe (IRAS17347-3139, IRAS18061-2505 and IRAS19255+2123) where the
physical conditions to generate water maser emission did not seem to exist.
These three objects may be considered as the youngest PNe known up to date and,
therefore, they are key objects to understand the formation of PNe. In
addition, the existence of water maser PNe allow us to study every phase in the
AGB to PN transition using water maser emission which can be observed at very
high spatial and spectral resolution. In this paper we review the properties of
water maser emission in PNe, the existing observations of the three water maser
emitting PNe and their implications in our understanding of PN formation and
evolution.
