This volume is based on a workshop "Modulation of Synaptic Transmission and Plasticity in Nervous Systems" held in n Ciocco, Castelvecchio, Pascoli, Italy, from September 8th to 13th, 1987. The purpose of the meeting was to bring together scientists working on plasticity in nervous systems on different levels. The contributions can be subgrouped into six fields of research: 1) Presynaptic Modulation of Chemical Neurotransmission 2) Postsynaptic Signal Transduction 3) Modulation of Synaptic Transmission and Plasticity in the Hippocampus 4) Modulation of Neuromuscular Transmission 5) Molecular and Cellular Analysis of Conditioning in Marine Snails 6) Analysis of Learning and Memory in Insects Understanding how nervous systems and in particular our brain processes and stores information has been a major challenge in science for centuries and will remain for some time to come. Not until recently neurobiologists agreed to seek plasticity of behavior primarily in the modulation of the properties of synapses between nerve cells. This is to be understood within the context provided by a neural circuitry. An important stimulus came from the work on the marine snail Aplysia, where learning processes can be described as a modulation of transmitter release, traced back to a complete chain of molecular events in an identified neuron. Learning became a topic of molecular biology. Three systems appear particularly promising for this approach: insects, in particular Drosophila, marine snails and the mammalian hippocampal tissue. Our views on neurotransmission have rapidly changed.