Pyrometallurgical extraction of metallic copper from copper concentrates generates several by-products, among which copper slag constitutes one of the most voluminous solid residues. These slags are generally composed of iron oxides, silica, calcium oxide, alumina, and varying amounts of unrecovered copper. Despite their common disposal by stockpiling or use as landfill material, copper slags may still host appreciable quantities of valuable metals occurring as entrapped metallic particles, sulphide phases, or oxidized compounds. For this reason, different recovery routes have been investigated and applied, including mineral processing methods as well as pyro-, hydro-, and combined pyro-hydrometallurgical treatments. Among these approaches, flotation is particularly important in copper slag beneficiation and is already integrated into several industrial smelting circuits. This review examines the characteristics of smelting, converting, refining, and ancient copper slags formed during pyrometallurgical copper production, together with the main strategies proposed for recovering metals from these secondary resources. Selected examples from laboratory-scale studies carried out on various industrial copper slags from Turkey are also presented. In addition, the environmental implications of copper slag management and the sustainability aspects of slag treatment are considered.