Although organometallic compds. are ubiquitous in nature, synthetic organometallic compds. are generally considered to be toxic or non-compatible with biol. systems. Despite this perception the medicinal properties of organometallic compds., in particular organo-transition metal compds., have been probed for a long time and in the last few years the area has grown considerably. In this chapter the medicinal properties of organometallic compds. are reviewed, with notable applications in the treatment and diagnosis of cancer and in the treatment of viral, fungal, bacterial and parasitic infections.