In recent years, biodiesel as a sustainable, biodegradable, and non-toxic fuel has drawn considerable interest from academics, governments, and industries. However, some feedstocks have been found inefficient or unfeasible owing to their incredibly high cost and their usage mainly as food products. Waste cooking oil (WCO) is considered as the most effective biodiesel feedstock even with its drawbacks, such as its strong free fatty acid (FFA), and water content. The impurities present in WCO can affect the transesterification reaction and biodiesel properties. Apart from this phenomenon, the biodiesel obtained from WCO gives better engine performance and less emissions when tested on commercial diesel engines. This chapter provides a comprehensive overview of the pretreatment and the usage of WCO in the production of biodiesel using several methods, different types of reactors, and various types and amounts of alcohol and catalysts.