An instar is a developmental stage of arthropods, such as insects, between each moult (ecdysis), until sexual maturity is reached. Arthropods must shed the exoskeleton in order to grow or assume a new form. Differences between instars can often be seen in altered body proportions or changes in the number of body segments. Some arthropods can continue to molt after sexual maturity, but these subsequent molts are generally not called instars.

For most insect species the term "instar" is used to denote the developmental stage of the larval or nymphal forms of holometabolous (complete metamorphism) or hemimetabolous (incomplete metamorphism) insects, but the term can be used to describe any developmental stage including pupa or imago (the adult, which does not molt in insects).

The number of instars an insect completes depends on the species and the environmental conditions. Cooler temperatures and lower humidity often slow the rate of development. Nonetheless, there seem to be some limits, and it is rare for an insect to pass through any fewer than 4 total larval instars before reaching maturity