Nitrogen oxide NO and the mixture of NO and NO2 indicated as NOx are present in cigarette smoke. These compounds are not listed as HPHCs by the FDA but they are commonly used in cigarette smoke. Both NO and NO2 have in their structure unpaired electrons and they may contribute to the formation of free radicals in smoke, which are also undesirable. The present chapter describes methods for the analysis of NO and NOx, usually based on detection using chemiluminescence.