A parasite lives in association with another organism (host), from which it obtains benefits and thereby harms.

Ichneumon fly, hatching from the optected pupa of an aphid.Strictly speaking, parasites and parasitoids must be differentiated:

• Parasites live upon or within a living host that survives the interaction.
• Parasitoids live upon or within a host, eventually causing it to die.

If a parasitoid is infected by a parasitoid itself, the latter is called a hyperparasitoid.

Parasites and parasitoids can affect hosts at various stages of development. They may lay eggs inside the eggs of their hosts, in host larvae or pupae, or even within the adult organism.

After predators, parasitoids are the most important biological control agents for insects (e.g. aphid).