Nowadays, the transformation of biomass into valuable chemicals and fuels through thermochemical, biochemical or even mixed technologies, is becoming increasingly popular and challenging. A promising solution for the near future is the substitution of non-renewable fossil fuels with a sustainable liquid feedstock for biofuel (biodiesel) production. The cost of conventional biodiesel production is higher than that of petroleum-based diesel production since it is produced mostly from expensive high-quality virgin oil. Conventionally, commercial biodiesel is produced via liquid base-catalyzed transesterification of triglycerides components of oil/fat with short-chain alcohols. It is that about 70–80% of the overall biodiesel production cost is associated with the cost of raw materials. Brown grease (with free fatty acid levels > 15%) is created from rendered trap waste and is known as Fats, Oils, and Greases (FOGs), it is a potential source of biodiesel feedstocks and is available at no cost. Many researchers are interested in using low-cost high Free Fatty Acid (FFA) oils as the feedstock for biodiesel production. This paper reviews the effect of feedstock pre-treatment and process parameters on the conversion of FOGs-wastewater to biodiesel by esterification-transesterification process.