Epidemic in Northern California Genome sequencing of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) outbreaks is valuable for tracing the sources and perhaps for drawing lessons about preventing future outbreaks. Genomic analysis by Deng et al. revealed that Northern California experienced a complex series of introductions of the virus, deriving not only from state-to-state transmission but also from international travel by air and ship. The study highlights the importance of being able to rapidly test and trace contacts of positive cases to enable swift control. Science , this issue p. 582