Crude oil can be extracted from the reservoirs by three mechanisms with different amounts of oil depending on the natural conditions of the reservoir. When the reservoir has an enough pressure, the amount of the produced oil is about 20% to 30% through primary recovery mechanism, and this amount can extent up to about 40% using secondary recovery. Because of the massive amount of the oil left behind the two mechanisms, enhanced oil recovery technique (EOR), the third mechanism, is designed to reduce the residual oil, in which, up to 70% of original oil in place can be recovered (Ahmed, 2018). Almost 3.0 trillion cubic meter light oil and 8.0 trillion cubic meter of unconventional oil will be left underground after primary and secondary stages (Thomas, 2008). Therefore, EOR techniques are applied to improve the oil production and extract much of the oil left in the reservoirs. Economics and technology have to be taken into account to choose the appropriate method in the recovery processes. Mainly, this study discusses various EOR techniques used in the enhancement of the oil recovery, including miscible, immiscible, polymer, surfactants, surfactants-polymer flooding as well as thermal methods.