It is now generally accepted that the vertebrate brain continues to produce new neurons throughout life, but it has not been clear whether new neurons are essential for memory formation. In the 15 March Nature, Tracey Shors of Rutgers University, New Jersey, and colleagues present evidence that newly generated neurons in the hippocampus of adult rats are essential for at least one form of memory — the kind concerning the timing of learned responses and temporal relationships between events (Nature 2001, 410:372-376).