Nerve growth cones are the motile tips of elongating axons and dendrites. The migration and behavior of growth cones are responsible for axonal pathfinding to synaptic targets, and for the branching patterns of dendritic trees. Growth cones are studied at all levels of organization, molecular, cellular, and within intact embryos. Preparations of growth cone fragments from neural tissues or neural cell lines are being used to identify the molecular components of growth cones. Sophisticated videomicroscopy and image processing are revealing the structure, intracellular motility, and metabolism of living growth cones at high resolution. Similar methods permit recording of growth cone activity within intact neural tissues, even complex vertebrate neural structures. These techniques and their findings were the focus of a recent meeting, honoring the discovery of growth cones in 1890 by the Spanish neurobiologist Santiago Ramon y Cajal. The speakers at this meeting wrote chapters for The Nerve Growth Cone, a thorough presentation of growth cone biology [Letourneau et al., 19911. Information in this monograph, as well as recently published papers, has provided several current issues for discussion. Rather than focus on the cytoskeleton and the assembly, disassembly, and organization of actin and tubulin polymers in growth cones, we will emphasize recent studies pertaining to how growth cones interact with their environment. The topics include a new aspect of growth cone behavior, the protein GAP-43 as a specialized component of growth cone motility, second messengers that may regulate growth cone motility, and signal transduction systems that link external cues to the machinery of growth cone migration. For another perspective on growth cone behavior see the recent Views and Reviews by Heidemann and Buxbaum [ 19901. BEING POSITIVE ABOUT NEGATIVE BEHAVIOR!