Octopamine (OA), a biogenic monoamine structurally related to noradrenaline, acts as a neurohormone, a neuromodulator and a neurotransmitter in invertebrates. It is present in relatively high concentrations in neuronal as well as in non-neuronal tissues of most invertebrate species studied. It functions as a model for the study of modulation in general. OA modulates almost every physiological process in invertebrates studied so far. Among the targets are peripheral organs, sense organs, and processes within the central nervous system. The known actions of OA in the central nervous system include desensitization of sensory inputs, influence on learning and memory, or regulation of the 'mood' of the animal. Together with tyramine, OA it is the only neuroactive non-peptide transmitter whose physiological role is restricted to invertebrates. This focussed the interest on the corresponding OA receptors. They are believed to be good targets for highly specific insecticides as they are not found in vertebrates. All octopamine receptors belong to the family of G-protein coupled receptors. Four of them could be distinguished using pharmacological tools. They show different coupling to second messenger systems including activation and inhibition of adenylyl cyclase, activation of phospholipase C and coupling to a chloride channel. Recently, octopamine receptors from molluscs and insects have been cloned. Further studies of all aspects of octopaminergic neurotransmission should give deeper insights into modulation of peripheral and sense organs and within the central nervous system in general.