Following the successful format of the first volume on long- term potentiation -- a leading candidate for the neuronal basis of learning and memory -- Volume 2 brings together the most recent data and hypotheses by top neuroscientists regarding the mechanisms of this phenomenon and of long-term depression (LTD).The book is divided into several sections covering different aspects of the field ranging from molecular mechanisms of plasticity to computational neurobiology. It revisits some of the major points covered in Volume 1, updating them in this fast-moving field. It also introduces several new issues that have arisen since then. Of the many possible new topics that could have been added, the editors have focused on retrograde messengers and the mechanisms and functions of LTP and LTD because they are the subject of much interest, research, and controversy. The section on retrograde messengers deals primarily with nitric oxide.