Biogenic monoamines are involved in virtually all functions in invertebrates. Octopamine and tyramine play major neurotransmitter, neuromodulator, and neurohormonal roles and are considered to be invertebrate counterparts of the vertebrate adrenergic transmitters. Furthermore, as in vertebrates, monoamines dopamine, serotonin, and histamine are involved in many vital functions of invertebrates. This article discusses advances in the understanding of biochemical pathways involved in synthesis and metabolism of monoamines, regulation of presynaptic mechanisms, specific receptors activated by these transmitters, and physiological functional roles of monoaminergic transmission in invertebrates.