We propose to investigate the far-IR emission characteristics of dust
 grains in planetary nebulae (PN) by using ISO to map the spatial
 distribution of the emission at three wavelengths.  Little is known of
 the properties of cool dust in PN or of its spatial extent in the
 ionized shell and halo.  Among the unanswered questions are whether the
 dust is heated by Lyman-alpha emission or by stellar continuum
 radiation.  Modeling these sources with a knowledge of the spatial
 distribution of the emission will help resolve this issue.
 Determining the cool dust characteristics will also give a better
 understanding of the mass loss history of the star that has formed the
 PN, and the dust that is returned to the ISM.  This study cannot be
 done from ground-based observations; ISO is required to provide the
 sensitivity and spatial resolution to carry out these observations.