ISO Science Operations Centre Astrophysics Division, Space Science Dept. of ESA Villafranca, Spain
The presence of dust particles residing at very low temperatures
(
)
has been hypothesised for a long time. The predictions
were based on assumed properties of dust particles in the infrared
combined with plausible conditions of the ISM in dense regions.
Nevertheless, our observational picture of the cold dust component
in the ISM is still poor despite the accessibility of photometric
windows in the wavelength range between 100 and 1000 
where
cold dust emission should be detectable.
The possible reasons why a very cold dust component should be
(prominently) present in the ISM are reviewed. The observational
evidences from ground based, space- and balloon-borne measurements
of Galactic and Extra-Galactic objects are discussed in detail.
It appears that the temperature distribution of cold dust in the
ISM in general or in dense condensations in particular is not
smooth but shows discrete components. The implications in terms
of dust properties, inferred dust mass, and cloud structure are
analyzed.