Plants utilized for agricultural productions interact with insects, fungi, and bacteria under the field conditions, affecting thereby their productivity. Since chitin and its derivatives play important roles in the interactions between these organisms, chitin-related enzymes are effective tools or drug targets for controlling the interactions. Thus, the molecular biology, protein chemistry, and enzymology of the chitin-related enzymes have been intensively studied by many investigators. Identifications and classifications of the genes encoding chitin synthetases, chitinases, chitosanases, and chitin deacetylases in these organisms were conducted, and their physiological functions were defined by knockdown, knockout, or overexpression of the corresponding genes. Recombinant enzyme productions and mutation studies are also being conducted to understand their structure and function. All of these studies have opened the way to efficiently utilize these enzyme tools for enhancing the agricultural productions.