Although calcareous algae are known both in present environments and fossil records, from shelf and upper slope settings, either in warm seas for green algae or at all latitudes for red algae, we still need models to quantify their abundance in space and time. Calcareous algae are an important component of biogenic carbonate production but they are very sensitive to marine acidification and rise in temperature, as illustrated by the effects of ongoing global change. Contributions herein were first presented at the 6^th Regional (European) Symposium of the International “Fossil Algae” Association: they include two reviews respectively devoted to the carbonate production of red and green algae, and a suite of investigations covering quantification, facies delineation, and controlling factors spanning the present-day Mediterranean Sea and Eastern Pacific going back to the Jurassic of Romania.