Carrageenans are sulfated polysaccharides derived from the extracellular matrix of red algae. Carrageenan oligosaccharides, the degradation products of carrageenan polysaccharides, possess the enhanced solubility, bioavailability, and bioactivity. They have some pharmaceutic values depending on their structures and modifications. Carrageenase can cleave the internal β-1,4 link in the carrageenan polymer and have broad applications. In this chapter, the enzymatic characteristics of different carrageenases are summarized, and their structural characteristics and catalytic mechanisms are discussed. In addition, different approaches to improving the enzyme properties of carrageenase are outlined, and the applications of carrageenases in biological studies are presented. This chapter provides a comprehensive description of carrageenases, which is significant for carrageenase exploration and application.