Although all the early telescope mirrors were made of speculum metal, once the chemical process for metallizing a glass surface became known in about 1850, glass replaced metal as the most common substrate material. However, there are now new applications, such as in the high energy laser field, where the high damage threshold and potential lower fabrication costs make the use of metal mirror substrates appealing. These new applications, coupled with recently developed diamond-turning fabrication techniques, have, during the past five years, made the field of metal optics extremely interesting, and the potential developments for the next five or ten years are even more exciting. The purpose of this special issue on "Metal Optics" is to summarize the reasons for the revival of the use of metal substrates. The issue describes the basic properties of metal mirrors and the most modern fabrication and testing techniques, as well as some of the applications.