Understanding the sources of background ozone is of importance as the ozone air quality standards become more stringent. The North American background ozone, defined as surface ozone concentrations in the absence of North American anthropogenic emissions, provides a baseline for risk and exposure assessments in setting the air quality standard. It represents the maximum ozone reduction that would be achieved through North American emission controls. Sources of the North American background ozone include intercontinental ozone transport, as well as natural sources from the stratosphere, lightning, and wildfires. The presentation will discuss transpacific transport of ozone pollution as seen from aircraft and satellite observations and interpreted with the GEOS-Chem chemical transport model. The presentation will also discuss the impacts of natural ozone sources such as stratospheric intrusions, lightning, and wildfires, and examine their uncertainties in the model estimation of the North American background ozone.