Recent advances in molecular neuroscience have revealed similarities in signaling mechanisms between and within cells among molecular events as diverse as drug addiction, olfaction, and nerve growth. This colloquium brings together leading researchers to discuss the panorama of signaling systems to look at the astonishing diversity of recently identified chemical messengers that convey information among neurons in the brain, as well as their major clinical consequences. From the Series:Brain Messengers: The Inaugural Arthur M. Sackler Lecture