Despite the fact that great progress has been made in developing photobioreactors for mass production of microalgal cells, more efforts are still required for further improvement, espe­cially regarding the cost reduction of bioreactor design. For large-scale outdoor microalgae cultivation, large amounts of required land space are still the critical issue. In addition, since outdoor photobioreactors usually utilize natural solar light and without additional temper­ature control, the growth of biomass would greatly depend on weather conditions and am­bient temperature. Due to these limitations, in most regions of the world it is not feasible to have stable microalgal biomass production through outdoor mass cultivation. In addition, the potential contamination is also a serious threat to the operational success of outdoor open ponds or raceways. In contrast, closed system photobioreactors have the advantages of better operational stability and condition control. However, the high equipment cost and process cost of closed photobioreactors are still barriers impeding the mass cultivation of microalgae. Finding more rigid, reliable, and transparent materials with lower costs for the design of closed photobioreactors is crucial to enhance cultivation efficiency and to reduce the cost of photobioreactors for the development of closed systems for the autotrophic cultivation of microalgae.