Nitrification used to be considered a relatively straightforward process. Most textbooks describe it as the oxidation of ammonia to nitrite and subsequently to nitrate by two groups of aerobic, autotrophic bacteria. The ammonia oxidising bacteria were originally placed in five genera and the nitrite oxidisers in four. A combination of physiological studies of pure cultures, sophisticated process measurements and the application of molecular techniques has demonstrated this view to be a vast oversimplification. Both ammonia and nitrite oxidising bacteria can utilise (but not necessarily grow on) organic substrates, both can maintain activity under low oxygen or anaerobic conditions and nitrifier communities display considerable diversity. These complexities, in themselves, have significance for soil nitrification. For example, utilisation of urea rather than ammonia enables nitrification under acidic conditions and denitrification by ammonia oxidisers may lead to significant rates of nitrous oxide production. The diversity of nitrifiers may also be reflected in physiological diversity, with consequences for resilience and robustness in the face of environmental change. The traditional view of nitrification has also been challenged by the discovery of previously unsuspected processes and microbial groups. Heterotrophic nitrification by bacteria and fungi may be important in certain environments and anaerobic nitrification introduces a new process carried out by planctomycetes, rather than the betaproteobacterial autotrophic ammonia oxidisers. More recently, metagenomic studies predicted the involvement of non-thermophilic crenarchaea in ammonia oxidation. The isolation of an ammonia oxidising, autotrophic, non-thermophilic crenarchaeon in laboratory culture confirmed the potential for archaeal ammonia oxidation and there is now evidence that ammonia oxidising archaea are more abundance and active in some soil environments. Our understanding of nitrification and nitrifiers has therefore undergone a series of revolutions which have considerable implications for the soil nitrogen cycle, the control of nitrification and the methods used for its study.