This chapter introduces working definitions of neuropeptides and neurotransmitters from the perspective of invertebrate physiological processes. Neuropeptides and neurotransmitters are intercellular chemical signaling agents used by all animals. Chemical signaling augments or substitutes for electrical communication in the nervous system. When these agents act as neurotransmitters, they convert electrical signals to chemical signals across the synapse. As hormones, they circulate from a site of release to act at a more distant site in the body of the organism. Neuropeptides and neurotransmitters are classified into these groups mostly on the basis of their molecular size. This article describes several neuropeptide superfamilies and their wide scope of actions in model invertebrates. The article also describes the main neurotransmitters used by invertebrates.