PBO is a distributed observatory of high-precision geodetic instruments designed to image the ongoing deformation of western North America. The geodetic network will extend from the Pacific coast to the eastern edge of the Rocky Mountains, and from Alaska to Mexico. Two complementary instrumentation systems—Global Positioning System (GPS) technology at about 1000 sites, and ultra-low-noise strainmeters at 200 locations—will provide superior time resolution. A sparse GPS network at 100-200 km spacing, connecting about 20 dense clusters located at active volcanoes and the most active earthquake faults, will provide complete spatial coverage. PBO will significantly improve our ability to image and characterize the slow deformation that accompanies earthquakes and volcanic events. These data will elucidate the poorly understood processes that precede earthquakes and volcanic eruptions, and could provide a firm basis for their prediction. The broad geographic coverage will permit quantitative understanding of tantalizing connections observed between activity in different regions. Complementary geological investigations will link present-day deformation to longer-term processes that have shaped the plate-boundary region.