The main purpose of this study was to investigate the extraction of essential oils from plants by the supercritical fluid extraction technique. However, other interesting information, regarding the process optimization, is also discussed. Therefore, logical steps were identified to completely describe the process. These steps were divided into three parts: (1) characterization of essential oils; (2) properties attributed to the supercritical fluid, including supercritical fluid extraction, parameters affecting the procedure such as temperature, pressure, particle size, modifier, flow rate, and water, the importance of fractionation, types of collection methods, the antimicrobial and antioxidant activities of essential oils extracted by supercritical fluid, advantages and drawbacks of this method in comparison with other extraction techniques; and (3) the use of experimental designs, including screening, optimizing and modeling, in the process of supercritical fluid extraction of essential oils.