Nitrification is the biological oxidation of ammonia with oxygen into nitrite followed with the oxidation of these nitrites into nitrates. Nitrification is an important step in the nitrogen cycle.

The oxidation of ammonia into nitrite, and the subsequent oxidation to nitrate is done by two different bacteria. The first step is done by bacteria of (amongst others) the genus Nitrosomonas . The second step (oxidation of nitrite into nitrate) is (mainly) done by bacteria of the genus Nitrobacter . All organisms are autotrophs, which means that they take carbon dioxide as their carbon source for growth.

Nitrification plays an important role in the removal of nitrogen from municipal wastewater. The conventional removal is Nitrification, followed by denitrification. The cost of this process resides mainly in aeration (bringing oxygen in the reactor) and the addition of an extra organic energy source (e.g. methanol) for the denitrification.

In most environments both organisms are found together, yielding nitrate as the final product. It is possible however to design systems in which selectively nitrite is formed (the sharon process).