Although cyanobacteria and algae represent a small fraction of the biomass of all primary producers, their photosynthetic activity accounts for roughly half of the daily CO fixation that occurs on Earth. These microorganisms are able to accomplish this feat by enhancing the activity of the CO-fixing enzyme Rubisco using biophysical CO concentrating mechanisms (CCMs). Biophysical CCMs operate by concentrating bicarbonate and converting it into CO in a compartment that houses Rubisco (in contrast with other CCMs that concentrate CO via an organic intermediate, such as malate in the case of C CCMs). This activity provides Rubisco with a high concentration of its substrate, thereby increasing its reaction rate. The genetic engineering of a biophysical CCM into land plants is being pursued as a strategy to increase crop yields. This review focuses on the progress toward understanding the molecular components of cyanobacterial and algal CCMs, as well as recent advances toward engineering these components into land plants.