Measurements by a number of investigators show that urban centers often increase CCN concentrations both locally and at distant sites through advection by factors of two or more over concentrations at rural locations (Hudson and Frisbie [1991], Montanez and Garcia-Garcia [1993]). Alkezweeny et al.[1993] showed that the cloud droplet size distribution in clouds containing anthropogenic CCN were significantly different. The effect of the urban areas was to increase the number concentration of droplets and decrease the median volume diameter and the variance of the droplet diameters. Clouds formed in unpolluted air had a median volume diameter of 21.9 microns and a standard deviation of 7.4 microns, while clouds formed in polluted air had a median volume diameter of 13.6 microns and a standard deviation of 1.9 microns. A study by Hudson et al. [1991] showed that forest fires resulted in large increases of CCN, while a jet fuel fire did not. On the other hand, Parungo et al. [1992] suggested that the particles produced by the Kuwait oil fires produced enhanced concentrations of CCN due to sulphate particles coating the hydrophobic surfaces of soot and dust particles. Studies of jet engine exhaust by Pitchford et al. [1991] showed that background CCN were much more active as CCN than exhaust particles of the same size, of which less that 1:100 are active at 0.8% supersaturation. In a recent study, Frisbie and Hudson [1993] showed that the spectral flux of CN (condensation nuclei) and CCN from Denver increased by a factor of 3 from 1966. These results suggest that the CCN concentrations are likely to continue to increase over time world-wide as the global population continues to increase. Studies of the effects of anthropogenically produced CCN on climate will obviously become increasingly important.