Denitrification is a process occurring in natural environments, such as soil, marine and fresh waters, where the role of the nitrogen cycle is relevant. Denitrificiation is defined as an anoxic biological process, which involves the reduction of nitrate or nitrite to molecular nitrogen. In this book, the authors present topical research in the study of the processes, regulation and ecological significance of denitrification. Topics discussed include the biological removal of nitrogen compounds from wastewaters; biofilm reactors in denitrification processes; design ecological treatment systems for nitrogen removal through nitritation and anammox; microbial water denitrification stimulated by electric fields and denitrification in plant-beneficial bacteria.