Musk compounds are widely spread environmental pollutants. Musk compounds were found in aquatic organisms from the North Sea, in rivers, lakes and estuaries in Canada, Czech Republic, Germany, Italy, Luxembourg, Japan, Norway, Switzerland, Sweden, and The Netherlands. Two nitro musks, musk xylene (MX) and musk ketone (MK), and two polycyclic musks (HHCB and AHTN) were the major musk compounds determined and observed in freshwater as well as in marine organisms. The main source of nitro and polycyclic musk residues in aquatic organisms are effluents from sewage treatment plants (STPs). The presence of synthetic musk compounds in biota can, therefore, be used as an indicator of the exposure of biota to STP effluents. Synthetic musk compounds have mainly been determined in fish, but some data are also available for mussels and shrimps. In addition, MX was found in eggs of coastal bird species, and HHCB and AHTN were identified in otters. The concentrations of HHCB and AHTN in freshwater organisms from Europe are one to two orders of magnitude higher than MX and MK, and comparable to levels of PCBs in fish. Indications were found that several fish species such as eel (Anguila anguila) could metabolise HHCB and AHTN, and that food chain transfer of these musk compounds from prey fish (e.g. roach) to carnivore fish (pike-perch) occurs. Time trend data for MX and MK in eel from the river Elbe (Germany) showed that for some locations a decline in the concentrations from 1994 to 1999 occurred, probably due to the restriction of the use of MX in Germany since 1993. Similar results were observed for MX in eel from the river Rhine. Nitro musks and polycyclic musks were found in fish samples (e.g. trout, herring, mussels, tuna and mackerel) collected at food markets. In some samples (trout and shrimp) the concentrations of MX and MK were similar to the concentrations of PCBs, while in other samples (e.g. halibut and mussels) the concentrations of MX and MK are one to two orders of magnitude lower than those of PCBs. In general, the highest concentrations of MX were found in trout, and in some tuna samples rather high concentrations of MK were found.