This paper critically discusses the structure, properties and applications of ironmaking and steelmaking slags and their silicate-based variants as low-cost adsorbents for removing cations and anions from industrial effluents and wastewater. Undoubtedly, the performance of slag-based adsorbents depends on their physical, chemical and phase chemical properties. The presence of crystalline phases, for example, has a significant effect on the adsorption capacity. However, despite their low cost and ubiquity, their chemical and geometric heterogeneity significantly affects the performance and applications of slag-based adsorbents. These challenges notwithstanding, the efficacy of slag-based adsorbents can be significantly enhanced through purposeful activation to increase the specific surface area and density of adsorption sites on the surfaces of adsorbent particles. The synthesis of functionalised adsorbents such as geopolymers, zeolites and layered double hydroxides from silicate and aluminosilicate precursors can also significantly increase the performance of slag-based adsorbents. In addition, the ability to stabilise the dissolved and/or entrained toxic metal species in stable phases in slags, either through controlled post-process fluxing or crystallisation, can significantly enhance the environmental performance of slag-based adsorbents. Most critical in the design of future slag-based adsorbents is the integration of the engineered properties of molten and solidified slags to the recovery and stabilisation of dissolved and/or entrained metals.