Calcareous algae are one of the most important reef formers, including the members of Cyanophyta, Rhodophyta, Chlorophyta and Phaeophyta. The main crystalline calcium carbonates of calcareous algae are calcite and aragonite forms. The basic requirement of calcification is an isolating environment. However, there are many hypotheses for calcification mechanisms, e.g., CO2 utilization theory, HCO3- usage theory and organic matrix theory. Calcification is important to the adaptive significance of calcareous algae. The hermatypic roles of reef formers include cementers, structural element formers and sediment formers. Except the role of structural element formers is assigned to corals, the other two roles are calcareous algae.