European legislation requires that special aggressive pollutant gases (i.e. NO), must be measured continuously, particularly in locations with the suspected highest load risk. There are a number of oxides of nitrogen, including nitrous oxide (N2O), nitric oxide (NO), nitrogen dioxide (NO2), nitrogen trioxide (N2O3), and nitrogen pentoxide (N2O5), that are referred to collectively as NOx. The two oxides of nitrogen that are of primary concern to air pollution are NO and NO2. The monitoring of NO2 for regulatory purposes requires methods suitable for concentrations measuring in the range relevant for Europe, i.e. 0 - 500 mg/m3, with a sufficiently high accuracy. Different methods have been developed for the measuring of NO2 concentrations, some of which can only measure NO2, while others can also measure NO and/or NOx. These methods are referring either to standard methods or to acceptable one, with simple chemical and physical principles, either to methods based on sophisticated electronic techniques. In this paper are presented certain methods for measurement, monitoring and control of the nitrogen oxides, recommended at international level. Through these methods, can be known the quantity of NOx emissions from ambient air. This is the first step to establish appropriate methods to prevent and reduce air pollution.