Copper and cobalt are essential metals for the transition to renewable energy and various aspects of modern life. Their extraction from primary sources, such as ores, requires metallurgical separation from unwanted host materials, generating a substantial amount of waste. One type of this waste is copper smelting slag, with 39 million tonnes produced and discarded globally each year. These large quantities of slag occupy significant land space, often near residential areas, and pose a potential hazard by releasing contaminants into the environment. However, they also contain residual amounts of valuable copper and cobalt. To address the challenge of mitigating the negative impacts of this waste and exploring the potential commercial opportunities of the contained critical minerals, this study reviews global smelting slag production over the past 25 years, as well as the composition and technical reprocessing options. It provides a summary of the chemical and mineralogical characteristics of copper slag from various research and an extensive overview of the processing strategies for metal recovery from copper slag, including flotation, pyrometallurgy, and hydrometallurgy. The study demonstrates that a significant amount of smelting slag has been produced with considerable variation and complexity, representing a major potential resource for cobalt and copper metals. The overview of production trends and reprocessing techniques shows that while some effective options exist or are emerging, further research is required to enhance the reprocessing of smelting slag waste to create economic value, improve energy efficiency in metal production, increase the supply of critical metals, and reduce environmental impacts.