Anaerobic ammonia oxidation (Anammox) is one of the important discoveries in the field of environmental microbiology, and it plays an indispensible role in the nitrogen removal from wastewaters and the biogeochemical nitrogen cycle. Through review research progress in anaerobic ammonia oxidation, an Anammox special issue is published so as to find problems, explore applications and outlook developments. The special issue consists of reviews and original papers, mainly involving in the following aspects: i) enrichment of Anaerobic ammonia oxidation bacteria (AnAOB); ii) community analysis of AnAOB; iii) preservation of granular AnAOB sludge; iv) effect of organic matter on Anammox; v) application of Anammox process, etc.