The metallome is defined as the entirety of metal and metalloid compounds in cells, body fluids, or tissues of an organism and results from the interactions and functional connections between metal ions and DNA, RNA, proteins, and metabolites. Thus, the metallome can be regarded as a subcategory of the genome, transcriptome, proteome, and metabolome. Metallomics defined as the investigation of the metallome is strongly interrelated to the other "-omics" fields but with a focus on essential and toxic metals and metalloid compounds. Metallomics studies require not only the identification and quantification of these compounds but also the investigation of their functional connections between DNA, proteins, and metabolites. The metallome is highly dynamic and comprises an immense amount of diverse structures in permanent transformation. Therefore, metallomics in its primary meaning as a global study of the entire metallome is so far not feasible due to the limitations of the available analytical techniques. To date, metallomics focuses on the study of subgroups of the metallome such as selenoproteins and selenometabolites, different metal species, or the metallothiolome. The progress in metallomics toward a global metallome investigation will strongly depend on the development of analytical techniques and the integration of data by bioinformatic approaches.