Publisher Summary This chapter focuses on using algae for energy production, namely, the biomass production system. Microalgae cultivation possesses features common to bioreactors in general, such as thermal regulation, nutrient feeding procedures, pH regulation, and mixing for heat and mass transfer enhancement. However, a light supply is necessary for photosynthetic growth, with several consequences, in particular the need for a dedicated cultivation system emphasizing large illuminated areas. Unlike other more classical bioprocesses where mixing tanks display standard geometries, cultivation systems for microalgae are characterized by a broad diversity, ranging from open ponds (open systems) to photobioreactor technology. The advantage of microalgae is its solar production with higher surface productivities than plants, simultaneous consumption of inorganic carbon, allowing a null carbon balance exploitation, and possible production in closed systems, offering several advantages including an intensified, controlled production with very low environmental impact. The high biodiversity of microalgae means that a variety of energy-rich substances can be produced, such as hydrogen by water photolysis, lipids for biodiesel or biokerosene production, and sugars for biomass fermentation (methane) or gasification.