Deoiling hydrocyclones are a mature technology for oil removal from produced water. Since the introduction of this technology in the 1980s it has become ubiquitous in all oil producing regions. From its original intent – to treat water directly off the outlet leg of a production separator – many operators have tried to use deoilers for every water treating application. The lack understanding the operation and design of deoilers has led to misconceptions around their performance. Deoiling hydrocyclones can treat emulsions, high-pressure gas condensate water, high-turndown requirements, and work with pumps – however their fundamentals must still be adhered to for the system to work properly. Not all deoiler liners are the same and vendor experience is critical during evaluation of this technology.