This book provides the foundations for understanding the cellular and molecular basis for neuromodulatory effects. It illustrates key examples of the roles played by neuromodulation in sensory processing, neuromuscular transmission, generation of motor behaviors, and learning. The book also underscores the importance of information processing by the nervous system in future studies. For neurobiology students and researchers, the book summarizes a vast amount of research in the context of how these cellular mechanisms are used in systems of neurons. By spanning the levels of analysis from sub-cellular mechanisms through cellular properties and neuronal systems to behavior, the book provides a framework for understanding this rapidly advancing field. The language is accessible to anyone with a basic understanding of neurobiology, but covers topics in depth for those familiar with neuromodulation and information processing.