This book is a colorful journey into the fascinatingly diverse world of interneurons, an important class of highly heterogeneous cells found in all cortical neuronal networks. Interneurons are known to play key roles in many brain functions, from sensory processing to neuronal oscillations linked to learning and memory. The central aim of the volume is to provide new insights into the striking degree of cellular diversity found in interneuronal microcircuits. The book discusses the history of research into interneuronal variability, the developmental origins of interneuronal diversity, the functional roles of heterogeneity in neuronal circuits, contemporary interneuronal classification systems, and the genetic and homeostatic mechanisms that shape the degree of cell to cell variability within interneuronal populations. It elaborates on new ideas about interneuronal diversity that rest upon theoretical and experimental results, with arguments touching upon evolution, animal behavior, and the mathematical theory of small world networks. This engaging volume is invaluable to neuroscientists and others interested in how neuronal newtworks function; electrical engineers, computational modelers, and physicists interested in neuronal network theory; neurologists and psychiatrists working on mechanisms of neurological and psychiatric disorders; and students and trainees in all of these fields.