Despite 1-1/2 decades of control effort, the photochemical ozone problem continues to plague human society and ecology in the U.S. One reason alleged for the difficulty in achieving the established ozone air quality standard is that current understanding of the science underlying the problem is still insufficient. Scrutiny of existing empirical and theoretical/experimental evidence revealed imperfections in ozone air quality models and model application procedures. Latest chemical mechanism developments established the Carbon Bond Mechanism (CBM) and Carter-Atkinson-Lurmann-Lloyd (CALL) mechanisms as superior to others but still lacking in some respects. Precursor-related uncertainties are of consequence both for models requiring emission rate input and for models requiring ambient concentration input. Other factors affecting model predictions are the composition of VOC emissions and the amount and makeup of pollutants transported into the modeled atmosphere. Regional ozone air quality models are now available and procedures are being developed for use of such models in formulating regional ozone control strategies. The relative roles of 'hydrocarbons' (VOC) and nitrogen oxides (NOx) differ in the urban and the regional ozone formation phenomena.