We present observations of O3 and O3 precursors measured at mid boundary layer altitude during field campaigns in Nashville, Tennessee (1995), New York City, New York (1996), Phoenix, Arizona (1998), Philadelphia, Pennsylvania (1999), and Houston, Texas (2000). Ozone production rates P(O3) and their sensitivity to NOx and volatile organic carbons (VOCs) are calculated using observed concentrations as inputs to a steady state box model. City to city comparisons are made to illustrate common features of urban photochemistry and features that are unique to specific cities. Ozone production rates vary from nearly zero to 155 ppb h-1. Differences in P(O3) depend on precursor concentrations, namely, radical sources, NOx and VOCs. Under conditions where P(O3) is greater than 25 ppb h-1, there is a potential to produce enough same-day O3 to transform a typical regional background into a severe O3 episode. Six such cases were observed, in Nashville, Philadelphia, and Houston, with elevated O3 concentrations in the afternoon (130--211 ppb) following a morning in which P(O3) was 25--140 ppb h-1. High P(O3) occurs when NOx concentrations are 5--25 ppb and OH-VOC reactivity is above 5 s-1. These conditions are infrequent, and aside from a common dependence on calm winds, reasons vary from city to city. In Nashville, high P(O3) was observed during a stagnation event over downtown and under the circumstance that an air mass with a high concentration of isoprene encountered a NOx source. In Houston, NOx and light olefins are coemitted from petrochemical facilities leading to the highest P(O3)s of the five cities studied. High values of P(O3) did not occur in Phoenix because of low radical production, caused mainly by a dry atmosphere, and a low VOC to NOx ratio. The sensitivity of P(O3) to its precursors varied from NOx limited in rural samples to VOC limited near emission sources. In Philadelphia, and especially Phoenix, the highest O3 production rates occurred under VOC-limited conditions. In Nashville, New York City, and Houston the high-P(O3) samples were near the ridge line where P(O3) responds somewhat to VOC reductions but is insensitive to NOx changes.