Farmers in Australia apply between 80 and 200 kg N ba-1 to cotton, but only part of this nitrogen is taken up and used by the crop, and the remainder is lost :from the plantsoil system. Our work at Narrabri with 15N labelled fertilizer has shown that the lost nitrogen is emitted to the atmosphere as a result of biological denitrification, and that the losses can be considerable (up to 92% of the amount applied). In this paper we have been asked to address the following: (i) occurrence of denitrifieation (ii) the risks of denitrification, (iii) practices to reduce denitrification, and (iv) current research on this topic. Before discussing these issues we briefly describe the denitrification process so that the reader fully understands the nature of the problem and the discussion that follows.