Recently, national and international effluent standards have become more stringent, posing a significant challenge in the water treatment industry. Accordingly, treatment techniques with minimal energy consumption and maximal performance are urgently required for wastewater and water treatments. This topic was investigated from both technical and environmental perspectives to improve water and wastewater treatment techniques and enhance the quality of water bodies. This Special Issue (SI) has attracted investigations by researchers worldwide, including those from Australia, the United States, Finland, Turkey, South Africa, Oman, China, Japan, Malaysia, and Pakistan. In this SI, research and review articles propose and discuss efficient water and wastewater treatment techniques. We hope that the readers of Water can learn about new aspects of wastewater treatment using physicochemical, biological, and hybrid techniques. Finally, we hope that this SI will contribute to the United Nations’ Sustainable Development Goal 6, which is to ensure a secure water supply globally through cost‐efficient technologies.