This paper describes and analyses the information related to the concentration of molybdenum in natural waters. The importance of this element in these systems is related to its role as enzyme cofactor in the fixation of nitrogen in phytoplankton. In eutrophic lakes, it can be responsible for >80% of the annual fixation of nitrogen. The concentration of Mo in natural waters is low; in rivers it is almost 10 times lower than in seawater but, in some cases, it can increase due to contamination from mining activities. In lakes and wetlands, depending on the trophic conditions, Mo concentration can vary greatly, from 0.003 nM to 312.7 nM. The concentration of Mo in superficial seawater is approximately 100 nM. In oxic water, MoO(4)(2-)concentration shows a conservative behavior, but when there is anoxic stratification, it can decrease gradually toward deep water due to the formation of MoS42-, a species that may sediment with particulate material of organic origin and/or Fe oxyhydroxides.