Several literature reviews have been published on microbial enhanced oil recovery (MEOR). This paper updates the state of art in MEOR process and presents a summary of field projects. The most common practice technique of MEOR is cyclic stimulation treatment of production wells. Normally small amount of microbial solution injected in a single well and left to soak for a period of time before putting the well back on production. This process results in a limited volume of the reservoir being treated. This type of treatment usually applied because it is easy to implement, quick response and relatively inexpensive. The second technique is to apply microbial along with water flooding to improve both sweep efficiency and displacement efficiency. A number of projects have been conducted to improve oil recovery using MEOR technique. In the laboratory, bacteria have been shown to produce chemicals such as surfactant, acids, solvents and gases (mainly CO2) that can extensively contribute to improving displacement efficiency. Microorganism growth at substantial rate and some are capable of polymer production, which resulted in improving the volumetric sweep efficiency of the process and consequently improvement of oil recovery. MEOR process is friendly to the environment, which is additional plus to the process. In this paper a complete review of the current laboratory work and field projects will be presented in additional to reviewing the mechanism of the process in details.