The rewarding properties of drugs depend on their capacity to activate appetitive motivational states. Because the mechanisms underlying natural reward are an important life-sustaining process and strongly conserved throughout metazoan evolution, invertebrate models provide a powerful complement to the mammalian systems traditionally used in addiction research. A wide range of organizational complexity, combined with genetically manipulable, and relatively simple, accessible nervous systems, make invertebrates excellent models in which to explore general addiction principles. These include the role of natural reward systems in learning, the basic biological mechanisms of drug addiction, and the long-term effects of early drug exposure. The contributions to this e-book illustrate the current state of invertebrate addiction research. The chapters show that the reward circuits of invertebrate taxa are surprisingly sensitive to human drugs of abuse. Employing learning paradigms typically used in vertebrate studies (viz., conditioned place preference and operant, self-administration paradigms), invertebrates are shown to exhibit aspects of the addiction cycle from activational effects of common psychostimulants, sensitization with repeated application, to extinction, withdrawal, and reinstatement. This highlights the value of the comparative approach for both exploring conserved mechanisms underlying drug addiction and the utility of invertebrate models in seeking potential solutions.