In the 19th century, the medicinal applications of leeches prompted basic research into their neurobiology, reproduction, development, and anatomy; subsequently, leeches became an important model for understanding the nervous system. In this monograph, each chapter provides a narrative account of experimental work on a particular area of leech neurobiology, and explains its significance for the broader field of neuroscience. The appendices describe methods for maintaining and manipulating leeches in the laboratory and include an atlas of neurons in the leech Hirudo medicinalis. Extensively illustrated, this book is a classic in the field and is considered a “must read” for neuroscientists and those interested in leech biology. It has been out of print for many years; however, some recent inquiries have prompted us to reprint it and make it available at an affordable price.