Despite being known for a long time, microalgae are gaining importance in recent decades because of their high capacity to fixate atmospheric carbon, assisting in the reduction of global warming. Concurrently with carbon fixation, the production of compounds of commercial interest and reuse of industrial and domestic wastewater turned microalgal culture into one of the most commented-on subjects in the scientific community. In this chapter the photosynthetic metabolism of microalgae is described and discussed, with emphasis on carbon sequestration. The chapter covers the basics of growing algae, the most influential factors in CO2 fixation, and quantification methodologies. Practical data on rates of carbon fixation by microalgae are presented, especially those genera of algae with the greatest potential for industrial application: Spirulina, Chlorella, Haematococcus, Dunaliella, and Botryococcus. Finally, aspects of the growing global carbon market and the role microalgal technologies can play are also discussed.