At present only a small number of planetary nebulae are known to possess binary nuclei. Since approximately 2/3 of main sequence stars are members of binary or multiple star systems, one might expect a large fraction of PN central stars to have gravitationally bound companions. Additionally, late-type stars are more numerous, and due to their low luminosities would be difficult to detect by visual observational methods at distances where PN are typically found (≧ 1 kpc). (Only 5 known PN are thought to be nearer than 0.5 kpc). It is thus possible, and in our view probable, that a significant number of PN nuclei possess cool companions, hitherto undetected.