  Ice naturally forms in the disordered or ``amorphous'' state when accreted
from vapor at temperatures and pressures found in the interstellar medium and
in the frigid, low density outer regions of the Sun's protoplanetary disk. It
is therefore the expected form of ice in comets and other primitive bodies that
have escaped substantial heating since formation. Despite expectations,
however, the observational evidence for amorphous ice in comets remains largely
indirect. This is both because the spectral features of amorphous ice are
subtle and because the solar system objects for which we possess high quality
data are mostly too close to the Sun and too hot for amorphous ice to survive
near the surface, where it can be detected. This chapter reviews the properties
of amorphous ice, the evidence for its existence and its consequences for the
behavior of comets.
