Neuromodulators can resculpt neural circuits, giving an animal the behavioral flexibility it needs to survive in a complex changing world. This ability, however, provides parasites with a potential mechanism for manipulating host behavior. This paper reviews three invertebrate host-parasite systems to examine whether parasites can change host behavior by secreting neuromodulators. The parasitic wasp, Cotesia congregata, suppresses host feeding partly by inducing the host (Manduca sexta) to increase the octopamine concentration in its hemolymph. The increased octopamine concentration disrupts the motor pattern produced by the frontal ganglion, preventing the ingestion of food. Polymorphus paradoxus (Acanthocephalan) alters the escape behavior of its host, Gammarus lacustris (Crustacea), possibly through an effect on the host's serotonergic system. The trematode Trichobilharzia ocellata inhibits egg-laying in its snail host (Lymnaea stagnalis), partly by inducing the host to secrete schistosomin. Schistosomin decreases electrical excitability of the caudodorsal cells. The parasite also alters gene expression for some neuromodulators within the host's central nervous system. In at least two of these three examples, it appears that the host, not the parasite, produces the neuromodulators that alter host behavior. Producing physiologically potent concentrations of neuromodulators may be energetically expensive for many parasites. Parasites may exploit indirect less energetically expensive methods of altering host behavior. For example, parasites may induce the host's immune system to produce the appropriate neuromodulators. In many parasites, the ability to manipulate host behavior may have evolved from adaptations designed to circumvent the host's immune system. Immune-neural-behavioral connections may be pre-adapted for parasitic manipulation. Documentation; Publications; Nervous system; Nutrition; Endocrinology; Reproduction; Reproductive behaviour; Behaviour; Parasites diseases and disorders; Parasites; Helminth parasites; Insect parasites; Hosts; Insect hosts Trichobilharzia ocellata (Digenea); Literature review; Hormones; Molluscan hosts; Lymnaea stagnalis; Parasite neuromodulators role in host behavioural manipulation, review Polymorphus paradoxus (Acanthocephala); Literature review; Hormones; Crustacean hosts; Gammarus lacustris; Parasite neuromodulators role in host behavioural manipulation, review Lymnaea stagnalis (Lymnaeoidea); Literature review; Nervous system; Egg laying; Digenean parasites; Trichobilharzia ocellata; Parasite neuromodulators role in host behavioural manipulation, review Gammarus lacustris (Amphipoda); Literature review; Nervous system; Avoidance behaviour; Escape response; Acanthocephalan parasites; Polymorphus paradoxus; Parasite neuromodulators role in host behavioural manipulation, review Manduca sexta (Sphingidae); Literature review; Nervous system; Feeding behaviour; Hymenopteran parasites; Cotesia congregata; Parasite neuromodulators role in host behavioural manipulation, review Cotesia congregata (Braconidae); Literature review; Hormones; Neuromodulators; Lepidopteran hosts; Manduca sexta; Parasite neuromodulators role in host behavioural manipulation, review none