Open systems can be simply categorized into natural waters (lakes, lagoons, ponds) and artificial ponds or containers. The most commonly used systems include shallow big ponds, tanks, circular ponds, and raceway ponds (Suh and Lee, 2003). Open ponds are much easier to construct and operate than most closed systems. However, major limitations in open ponds include poor light utilization by the cells, evaporative water losses, diffusion of CO2 to the atmosphere, and the requirement of large areas of land. The ponds are usually kept shallow to ensure sufficient light exposure for the microalgae because sunlight can penetrate the pond water to only a very limited depth. Furthermore, contamination by predators, alien microalgae species, and other fast-growing microorganisms restrict the commercial produc­tion of algae in open culture systems. In addition, due to inefficient stirring mechanisms in open cultivation systems, their gas transfer rates are relatively poorer than those of closed systems. All these limitations lead to lower biomass productivities for open systems com­pared with those of closed systems. Nevertheless, the simple operation and easy scale-up for mass cultivation make open systems the first-choice option for microalgae cultivation in industrial applications.