The concentration Nd of cloud droplets in marine low clouds is a primary determinant of their ability to reflect sunlight and modulates their ability to precipitate. Studies have focused upon aerosol source variability as the key driver of variability in Nd. In this talk, I will explore the impact of both precipitation and source variability on Nd. I show that a simple aerosol budget model for the marine boundary layer can skillfully predict the geographical variability of Nd in regions of extensive marine low clouds. It is shown that drizzle is a key control on the Nd climatology. Drizzle results in reduction in Nd by factors of 2–3 over the remote oceans. Within 500 km of coastlines the reduction in Nd due to precipitation is weak but in these regions the model is not able to accurately predict Nd because of strong pollution sources. In general, both free-tropospheric and surface aerosol sources are likely needed to maintain Nd against precipitation losses. The results demonstrate that light precipitation rates typical of marine stratocumulus impact the radiative properties of marine low clouds not only by impacting cloud dynamics but by modulating marine boundary layer aerosol. Cloud droplet concentration should therefore be seen as a dependent variable of the system, and care must be taken to interpret its correlations with cloud microphysical and macrophysical variables as indicative of aerosol indirect effects.