A pupa is the life stage of some insects undergoing transformation. The pupal stage is found only in holometabolous insects, those that undergo a complete metamorphosis, going through four life stages; embryo, larva, pupa and imago. The pupae of different groups of insects have different names such as chrysalis in the Lepidoptera and tumbler in mosquitoes. Pupae may further be enclosed in other structures such as cocoons, nests or shells. In the life cycle of an insect the pupal stage follows the larval stage and precedes adulthood (imago). It is during the time of pupation that the adult structures of the insect are formed whilst the larval structures are broken down. Pupae are inactive, and usually sessile (not able to move about). They have a hard protective coating and often use camouflage to evade potential predators.

Insects emerge from pupas by splitting the pupal case, and the whole process of pupation is controlled by the insect's hormones. Most butterflies emerge in the morning. In mosquitoes the emergence is in the evening or night. In fleas the process is triggered by vibrations that indicate the possible presence of a potential host. Prior to emergence, the adult inside the pupal exoskeleton is termed "pharate". Once the pharate adult has eclosed from the pupa, the empty pupal exoskeleton is called an exuvium or exuvia; in most hymenopterans (ants, bees and wasps) the exuvium is so thin and membranous that it becomes 'crumpled' as it is shed.