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The extinction-to-backscatter ratio (S-a) is an important parameter used in the determination of the aerosol extinction and subsequently the optical depth from lidar backscatter measurements. We outline the algorithm used to determine S. for the Cloud and Aerosol Lidar and Infrared imager Pathfinder Spaceborne Observations (CALIPSO) lidar. S-a of the CALIPSO lidar will either be selected from a look-up table or calculated using the lidar measurements depending on the characteristics of aerosol layer. When lofted layers are encountered, S. is computed directly from the integrated backscatter and transmittance. In all other cases, the CALIPSO observables: the depolarization ratio delta, the layer integrated attenuated backscatter gamma, and the mean layer total attenuated color ratio chi, together with the surface type, are used to aid in aerosol typing. Once the type is identified, a look-up-table developed primarily from worldwide observations, is used to determine the S-a value. For aerosols in non-polar regions, CALIPSO aerosol models include desert dust, biomass burning, background, polluted continental, polluted dust, and marine aerosols. The polar aerosols are classified into three groups: arctic, polluted arctic, and antarctic aerosols.