Rhenium (Re), a very scarce element in nature was the last naturally available element to be discovered in 1925. Re is one of the rarest metals, found in the Earth's crust. The authors of this book provide a review of the discovery of rhenium and it's consequences. They also present an overview of the most significant applications of oxo-rhenium complexes as catalysts in organic chemistry, and the valorisation of mineral resources in Portugal.