Oil-in-water and water-in-oil both represent contaminations; and, the former is also a serious example of pollution that has no straightforward remedy. In fact, oil/water contamination is a serious global issue that has a very high impact on the environment and the ecosystem. Therefore, a reliable and efficient solution to this problem is of high significance in order to save our planet’s precious water resources. In this regard, materials featuring tunable wettability characteristic toward oil and water have found potential use in the separation of oil/water mixtures and emulsions. Keeping in mind the coverage of this book, this chapter presents a concise overview of the oil/water separation techniques that have so far been developed, including both conventional and advanced methods. This overview is especially focused on the principles, functions, materials and designs of the involved techniques, along with listing the definitions of different common terminologies that are prevalent in the area of oil/water contamination and separation.