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.