Since the recent paper by Beard and Ochs [1993] presents a comprehensive review of warm rain initiation through the development of large cloud droplets, we present only the results of their discussion in this section and refer the reader to their paper for further details. Two key areas were identified by Beard and Ochs [1993] as requiring additional research: 1) the role of giant and ultra-giant nuclei in warm rain initiation, and 2) better experimental data on the collection efficiencies of important droplet sizes. Laboratory data suggests that some of the theoretical collision efficiencies are too high, suggesting that collection may be weak in the absence of coalescence nuclei, leaving model calculations of warm rain development uncertain.