As part of the Hemispheric Transport of Air Pollution (HTAP; http:// www.htap.org ) project, we analyze results from 15 global and 1 hemispheric chemical transport models and compare these to Clean Air Status and Trends Network (CASTNet) observations in the United States (US) for 2001. Using the policy-relevant maximum daily 8-h average ozone (MDA8 O 3 ) statistic, the multi-model ensemble represents the observations well (mean r 2 =0.57, ensemble bias = +4.1 ppbv for all US regions and all seasons) despite a wide range in the individual model results. Correlations are strongest in the northeastern US during spring and fall ( r 2 =0.68); and weakest in the midwestern US in summer ( r 2 =0.46). However, large positive mean biases exist during summer for all eastern US regions, ranging from 10–20 ppbv, and a smaller negative bias is present in the western US during spring (~3 ppbv). In nearly all other regions and seasons, the biases of the model ensemble simulations are ≤5 ppbv. Sensitivity simulations in which anthropogenic O 3 -precursor emissions (NO x + NMVOC + CO + aerosols) were decreased by 20% in four source regions: East Asia (EA), South Asia (SA), Europe (EU) and North America (NA) show that the greatest response of MDA8 O 3 to the summed foreign emissions reductions occurs during spring in the West (0.9 ppbv reduction due to 20% emissions reductions from EA + SA + EU). East Asia is the largest contributor to MDA8 O 3 at all ranges of the O 3 distribution for most regions (typically ~0.45 ppbv) followed closely by Europe. The exception is in the northeastern US where emissions reductions in EU had a slightly greater influence than EA emissions, particularly in the middle of the MDA8 O 3 distribution (response of ~0.35 ppbv between 35–55 ppbv). EA and EU influences are both far greater (about 4x) than that from SA in all regions and seasons. In all regions and seasons O 3 -precursor emissions reductions of 20% in the NA source region decrease MDA8 O 3 the most – by a factor of 2 to nearly 10 relative to foreign emissions reductions. The O 3 response to anthropogenic NA emissions is greatest in the eastern US during summer at the high end of the O 3 distribution (5–6 ppbv for 20% reductions). While the impact of foreign emissions on surface O 3 in the US is not negligible – and is of increasing concern given the recent growth in Asian emissions – domestic emissions reductions remain a far more effective means of decreasing MDA8 O 3 values, particularly those above 75 ppb (the current US standard).