  Water is fundamental to our understanding of the evolution of planetary
systems and the delivery of volatiles to the surfaces of potentially habitable
planets. Yet, we currently have essentially no facilities capable of observing
this key species comprehensively. With this white paper, we argue that we need
a relatively large, cold space-based observatory equipped with a
high-resolution spectrometer, in the mid- through far-infrared wavelength range
(25-600~$\mu$m) in order to answer basic questions about planet formation, such
as where the Earth got its water, how giant planets and planetesimals grow, and
whether water is generally available to planets forming in the habitable zone
of their host stars.
