Insect mouthparts are modified appendages of head segments that are adapted to exploit different food sources. This chapter describes the general mouthpart morphology of Hexapoda, introduces basic feeding types in insects, and illustrates mouthpart function. Insect mouthparts include three appendages, the paired mandibles, the paired maxillae, and the unpaired labium as well as additional head structures, the labrum, and the hypopharynx. The noninsect lineages of Hexapoda possess entognathous mouthparts, which are concealed inside the head, while ectognathous mouthparts of Insecta articulate externally on the head capsule. Especially in winged insects, characteristic adaptations of mouthparts evolved in context with various food sources resulting in feeding specialization and enhanced functional performance. Insect mouthparts can be categorized in three principal functional types: (1) mandibulate biting and chewing mouthparts, (2) haustellate mouthparts forming variously composed proboscises, and (3) filter-feeding mouthparts of aquatic immature stages. The diversity of functional types and remarkable modifications are presented in various examples; characteristic patterns of mouthpart evolution are discussed. The composition of mouthparts in the various hexapod orders is summarized in a table. Additional functions, like defense, brood care, and male-male competition, modified the mouthparts in some insects. Rudimentary mouthparts are found in some nonfeeding adults of various insect taxa.