The mirror of the first reflecting telescope was polished by Newton in a speculum metal substrate. Subsequent inventions of practical methods of "silvering" glass diverted attention from metal substrates for many decades, but new applications, new environments, and improvements in manufacturing technology have forced a relatively recent (10-15 year time scale) return to more extensive use of metal mirrors. This paper surveys the basic properties of metals of practical utility for mirror substrates. Desirable combinations of property values, or figures of merit, for different environmental applications are discussed. The extrinsic properties, i.e. those which may be peculiar to a particular manufacturing process for a given metal, or combinations of metals, may also affect the choice of material for a particular application and some of these are briefly noted.