At the turn of the century, the pioneering neuroscientist Ramon y Cajal articulated the hypothesis that growth processes involved in the development of the central nervous system may persist into adulthood, where they might be retained to mediate the formation and maintenance of memory. Over the decades since Cajal s seminal suggestion, extensive experimental attention has been directed at elucidating the cellular and molecular mechanisms underlying both neuronal development and learning and memory. Many exciting technical and conceptual advances have been made on each front. Thus, as we...