Adsorption has been the most common technology used for water and wastewater treatment for a number of reasons including simplicity, flexibility in design, high efficiency and cost effectiveness. Although it has been a proven technology for removal of contaminants including heavy metals, it has a major limitation to its application, which is the cost of the adsorbent. In this context, natural clay minerals are considered as low cost materials because they are abundant and can be easily modified to improve their surface properties (e.g., surface area, charge) and consequently increase the adsorption capacity and a wide range of other applicabilities. The two most common modification methods of clay minerals are thermal modification and acid activation. The aim of this paper is to critically review the current status of thermally modified and acid activated clay minerals for environmental applications. It will evaluate the adsorption efficiencies and conditions for removal of contaminants and identify the existing data gaps to widespread commercial uptake of the technology.