From a traditional definition in the field of human health, metabolomics measures the concentrations of the large number of naturally occurring small molecules (called metabolites), that are produced as intermediates and end-products of all metabolic processes.They are measured from biological samples and body fluids such as urine, saliva, blood plasma, tissue sample; even the simple breath (exhaled breath condensates) can carry the information about the state of health.In environmental issues the same approach can be followed looking holistically to all small molecules detectable in a given systems in various scales, integrating thus metabolites from living organisms and all their biotic/abiotic transformation products.The total number of different organism metabolites is still unknown; some estimation ranges from a few thousands, up to 200,000 to about 1,000,000, but even this latter estimate may be conservative.Including plant and bacterial metabolites that are not necessary to keep the organism alive, also referred to as secondary metabolites, the number is enormously larger.