The Eta-Community Multiscale Air Quality (CMAQ) model's forecast performance for ozone (O3), its precursors, and meteorological parameters has been assessed over the eastern United States with the observations obtained by aircraft, ship, ozonesonde, and lidar and two surface networks (AIRNOW and AIRMAP) during the 2004 International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) study. The results at the AIRNOW sites show that the model was able to reproduce the day-to-day variations of observed daily maximum 8-hour O3 and captured the majority (73%) of observed daily maximum 8-hour O3 within a factor of 1.5 with normalized mean bias of 22%. The model in general reproduced O3 vertical distributions on most of the days at low altitudes, but consistent overestimations above ∼6 km are evident because of a combination of effects related to the specifications of lateral boundary conditions from the Global Forecast System (GFS) as well as the model's coarse vertical resolution in the upper free troposphere. The model captured the vertical variation patterns of the observed values for other parameters (HNO3, SO2, NO2, HCHO, and NOy_sum (NOy_sum = NO + NO2 + HNO3 + PAN)) with some exceptions, depending on the studied areas and air mass characteristics. The consistent underestimation of CO by ∼30% from surface to high altitudes is partly attributed to the inadequate representation of the transport of pollution associated with Alaska forest fires from outside the domain. The model exhibited good performance for marine or continental clear airflows from the east/north/northwest/south and southwest flows influenced only by Boston city plumes but overestimation for southeast flows influenced by the long-range transport of urban plumes from both New York City and Boston.