This section provides an introduction to the various types of nitrogen present in water and wastewater. In-line UV/persulfate digestion and oxidation with flow injection analysis (B) and the persulfate method (C) are discussed. The conductimetric method (D) is suitable for measuring ammonia in aqueous solutions. Conductimetric detection combined with a hydrophobic membrane provides for high sensitivity, rapid analysis (1-2 minutes per sample) and eliminates interferences related to color, turbidity and other ionic species present in the sample.