Chitinases are found in many organisms, and their properties seem to be closely related to their biological function. In this chapter, the physicochemical properties of chitinases such as molecular size are compared among organisms, and the optimum and stability conditions for chitinase activity are described. Furthermore, considering their classification based on amino acid sequence, kinetic behaviors are discussed together with their biological functions. In particular, hydrolytic mechanisms such as inversion and retention of the substrate are discussed in relation to allosamidin inhibition.