Clouds are an important component of air quality modeling and play a key role in aqueous chemical reactions, vertical mixing of pollutants, and removal of pollutants by wet deposition. Clouds also indirectly affect pollutant concentrations by altering the solar radiation, which, in turn, affects photochemical pollutants, such as ozone, and the flux of biogenic emissions. The Cloud module in CMAQ performs several functions related to cloud physics and chemistry. Three types of clouds are modeled by CMAQ: sub-grid convective precipitating clouds, sub-grid non-precipitating clouds, and grid-resolved clouds. The subgrid cloud scheme in the CCTM was derived from the diagnostic cloud model in RADM, Version 2.6 (Dennis et al., 1993; Walcek and Taylor, 1986; Chang et al, 1987; Chang et al., 1990). Grid resolved clouds are diagnosed by MM5 with no additional cloud dynamics considered in CMAQ. The aqueous chemistry model in CMAQ evolved from the original RADM model (Chang et al., 1987, and Walcek and Taylor, 1986). The cloud module in the CCTM vertically redistributes pollutants for the sub-grid clouds, calculates in-cloud and precipitation scavenging, performs aqueous chemistry, and accumulates wet deposition amounts. Version 4.5.1 contains an important improvement in the convective cloud mixing algorithm that corrects a tendency to over-predict excessive transport from upper layers in the cloud to sub-cloud layers.