The information presented in the Handbook of Brain Microcircuits was previously dispersed across the literature. In fact, some microcircuits were previously brought together for selected regions in The Synaptic Organization of the Brain edited by Gordon Shepherd (2003) and Microcircuits edited by Sten Grillner and Ann Graybiel (2006). This handbook greatly extends that coverage to over 40 regions of the vertebrate and invertebrate nervous system becoming the go-to source for key circuits within the neurosciences. In order to focus on principles, each chapter is brief, organized around 1-3 wiri