Today, 75 institutions get a warning from ShakeAlert before damaging ground shaking in the western United States. Legislation in California calls for a public system, and construction has recently begun with federal funding.  This is possible due to recent advances in our understanding of earthquake process coupled with new technologies to collect and distribute information and alerts.  The challenge is to provide more robust alerts making use of distributed networks of sensors.  Systems like ShakeAlert use high quality purpose-built networks of sensors distributed across a region.  However, noisy data is still an issue.  New developments suggest that smartphone sensor networks like MyShake could also contribute to the system, but they are even noisier.  The challenge is therefore developing techniques to analyze continuous and triggered real-time data to accurately detect and characterize earthquakes quickly enough to provide warnings for events around the world.