On average, two-thirds of the original oil in any reservoir remains unrecovered, even after water injection into the reservoir. The same is true if a low-pressure gas is injected into a reservoir which is largely immiscible with the oil. The oil is trapped in the reservoir due to the capillary forces and the interfacial tension, and remains entrapped regardless of how much water or low-pressure gas is injected into the reservoir. It forms either a discontinuous phase in the swept zone, or a continuous phase in the unswept zone of the reservoir. One way of enhancing the recovery of oil is by injecting into a reservoir a fluid which is miscible with the oil in place. In principle, the injected fluid should drastically reduce the capillary and interfacial forces. If that happens, then one may recover a large fraction of the trapped oil. For example, if one injects a gas (or a gas mixture) into an oil reservoir and if, at the temperature and pressure of the reservoir, the gas is in a critical or near critical state, it will mix in large proportions (if not completely) with the oil in place. Under these conditions, one will have an efficient miscible displacement process. For example, the critical temperature of CO2 is only about 31◦ C and, therefore, it can be an ideal agent for miscible displacement of oil. In fact, field-scale CO2 injection has been successfully carried out in the United States. It is such gas injection processes and the associated phenomena in oil reservoirs that are of prime interest to us in the present chapter. If we inject a gas into a reservoir which is completely or partially saturated with oil, and if the gas and the oil-in-place mix in all proportions, the gas and the oil are said