Insect chitinases belong to family 18 of the glycoside hydrolase superfamily (GH18) and comprise endo-splitting enzymes that retain the anomeric β-(1,4) configuration of the cleavage products. However, some of them have lost their catalytic activity but retained the chitin binding activity and/or possess imaginal disc growth factor activity. In all sequenced insect genomes, multiple genes encode chitinases, which are differentially expressed during development and in various insect tissues. Some of them have nonredundant functions and are essential for growth and development. A characteristic property is their multidomain architecture, which comprises varying numbers of catalytic and chitin-binding domains that are connected by glycosylated serine/threonine linker regions. Based on sequence similarities and domain organization, they have been classified into eight different groups. Insect chitinases have gained increasing interest for use in the biological control of parasites, fungi, and insect pests, and some enzymes have properties that make them highly attractive for biotechnological applications.