The process for using surfactants to effect the recovery of crude petroleum oil from underground porous oil fields is far from being realized. However, the concepts and the aims are important and have given rise to a great interest in the surfactants capable of producing ultralow surface tensions in surfactant-brine-oil systems. The purpose here is to illustrate the difficulties in recovering the oil and discuss how, with systems such as microemulsions, some of these difficulties may be overcome. Since the understanding of the behavior in such systems is only recent, the research on their applications to oil recovery is very new. Still some observations have been made which, although lacking a coherent form at present, are of considerable interest.