Metabolomics can map the large metabolic diversity in species, organs, or cell types. In addition to gains in enzyme specificity, many enzymes have retained substrate and reaction promiscuity. Enzyme promiscuity and the large number of enzymes with unknown enzyme function may explain the presence of a plethora of unidentified compounds in metabolomic studies. Cataloguing the identity and differential abundance of all detectable metabolites in metabolomic repositories may detail which compounds and pathways contribute to vital biological functions. The current status in metabolic databases is reviewed concomitant with tools to map and visualize the metabolome.Biological databases are indispensable for comparing genomes, proteins, and biological regulation. GenBank TM , Protein Data Bank (PDB), and Gene Expression Omnibus (GEO) are prime examples of how collecting biological information in a coherent manner enables novel insights into evolution and to derive testable hypotheses for gene function, yet biochemical databases on substrate-product relationships and organismspecific metabolic networks have lagged behind. Much of this lag is due to the inherent complexity of enzymology. Small changes in protein folding or in mutations in catalytic sites not only may change reaction kinetics but also have large impact on substrate specificity. Enzymes may have much broader substrate specificity than usually considered. Moreover, many enzymes exert reaction promiscuity (1), which is exploited in bioengineering but which also complicates the reconstruction of metabolic networks. Low-abundant enzymatic side reactions have likely not been reported in favor of the dominant and apparently biologically relevant functions and are consequently lacking in biochemical databases, yet such side reactions may become the major enzyme function through evolutionary pressure. Hence, the number of metabolites per species (or per cell type in multicellular organisms) is hard to predict except for the most conserved metabolic pathways.Accordingly, a surprisingly small fraction of detected metabolites can be readily identified by sensitive screening tools such as HPLC-or GC-coupled MS (Fig. 1). It appears that the metabolome is much larger than anticipated. Phenotypes of species need to be determined by their individual metabolic capacities, defined by the plasticity and flexibility of their metabolic networks. Metabolites can act as intracellular and extracellular signals at very low concentrations and enable communication between organs, as well as serve multiple and vital roles for species in their ecological niches, e.g. for defense or reproductive purposes.