There are many modes of communication that neurons use to transmit information besides what has come to be called neurotransmission. Many of these other types of communication can be classified as neuromodulatory, where instead of conveying excitation or inhibition, the signal from one neuron changes the properties of other neurons or synapses. This form of neuronal communication is often overlooked by systems physiologists, but it is extremely prevalent in the nervous system and needs to be included in any description of how the nervous system processes information. This book provides the foundations for understanding the cellular and molecular basis for neuromodulatory effects. It illustrates some key examples of the roles played by neuromodulation in sensory processing, neuromuscular transmission, generation of motor behaviours, and learning. Finally, the book seeks to point out areas that are likely to be of importance in the future study of information processing by the nervous system. For neurobiology students and researchers, the book summarizes a vast amount of research, and puts it into 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 behaviour, the book provides a framework for understanding this currently exploding field of research. It is accessible to anyone with a basic understanding of neurobiology, but is in depth enough to satisfy readers who are very familiar with the field.