Emergency room (ER) diversions-when ambulances are redirected from one hospital emergency room to another-are becoming common in communities across the country, raising concern that critically ill patients are increasingly confronting obstacles to timely medical care. Although hospitals have long diverted patients during the winter flu season, recent site visits conducted by the Center for Studying Health System Change (HSC) reveal that ER overflows are now a year-round problem. As this Issue Brief describes, difficulty obtaining emergency services may be just the most visible evidence of deeper problems facing many hospitals as they struggle to meet growing demand for services at a time of increasing capacity constraints.