Dipterean insecta owe their relevance to the well-known model organism D. melanogaster, and extensively to the Drosophila genus. In the frame of the study of metallothioneins (MTs), they constitute a bizarre exception, since Drosophila is the only metazoan organism synthesizing only MTs similar to the yeast (S. cerevisiae) Cup1 paradigmatic copper-thionein. D. melanogaster MTs are optimized for copper binding and metabolism, and no MT analogous to the mammalian MTs has been identified, unlike in the cases of other Arthropoda and invertebrates analyzed so far (Chapters 7 and 8 of this book). The D. melanogaster genome contains four MT genes (MtnA, MtnB, MtnC, and MtnD), which are clustered in the same genomic region, probably arising from amplifying duplications. The main isoforms are MtnA and MtnB, encoding peptides 40- and 43-amino acids long, comprising 10 and 12 cysteines, respectively. MtnC and MtnD probably arise from recent MtnB duplications and play a minor role in flies. Expression of the four genes is dependent on dMTF-1, a transcriptional activator homologous to the mammalian MTF-1. Major MT synthesis in larvae and adults is observed in the digestive tract, mainly in the midgut, although with a different distribution between MtnA and MtnB. Other tissues of Mtn gene expression are salivary glands, ventricula, Malpighian tubules, and hemocytes. Metal induction increases the rate of MT synthesis in these tissues, but not normally in MT gene-silent organs, and invariably, the best response is obtained for copper, followed by cadmium, and very poorly for zinc. In conclusion, all the available data suggests a definitive role of D. melanogaster MT in copper metabolism (from ingestion to distribution, storage, delivery, and detoxification in the organism) as well as in cadmium tolerance (by digestive assimilation blockage). This is readily confirmed by studies of population genetics that show that both spontaneous MtnA genomic duplications, independently isolated in worldwide locations, and fly strains containing supernumerary Mtn genes constructed in the laboratory are able to tolerate increased copper and cadmium concentrations; while Mtn and dMTF-1 knock-outs are extremely sensitive to these conditions.